I've been wanting to write this entry for a very long time, but it is a bit of an emotionally charged subject for me. So here it goes.
Terminal length put plainly means the maximum length your hair is able to reach. If you want to be technical about it, terminal length happens when your shedding rate equals your growth rate. Now I know this sounds very confusing, but I'm going to explain it plainly. The first time I read that above sentence I was awash in a sea of confusion lol, honestly. Ok so your hair sheds, everyone's hair does this; and your hair grows, everyone's hair does this as well. Now each of the hairs on your head grows at different speeds and each of them sheds after a different amount of time, which means you have all kinds of length hair on your head at any given time, unless you shaved it all off. For example the hairs around my hairline shed after growing for about 3-6 inches, but that's not true for the hairs in the middle; and if I don't trim my hemline is uneven because each hair grows at a different speed (this is also referred to as fairy-tale ends and you can see them a lot in anime characters, it is also noticeable when growing out hair dye). Now when you reach your terminal length that means the hairs on your head that wait the longest amount of time to shed and grow the fastest are done growing and will now shed. A false terminal length happens when you don't trim and your ends just crumble over and over again, but if you trimmed you would reach new and exciting lengths. Cinnamon reached a false terminal at hip length and managed to grow her hair to knee length with trims: Growing Out Thin Ends -- Success Story!.
The way terminal length looks differs from person to person when it comes to hemline shape, it could be a U or V shape; or a reversed U, very seldom is it reported to have more of a blunt look. Men tend to get the V shape and women tend to get the reversed U, this is because women tend to have the longer growing hairs in the top of their scalp, while men tend to have it on the edges of the scalp (notice how men who bald tend to have hair only on the sides and lose the hair on the top and front of the head).
All of the hair should taper gradually to a point. That means that whatever your ponytail circumference is, the amount of hair will decrease more and more until only a few hairs reach the longest length, usually only 2-5 hairs will reach that ultimate length and even if you trim, your hairs will never get beyond that length. This is also a way to measure what your terminal length will be. If at the base your ponytail measures 3 inches, then 10 inches down the length it is 2 inches in thickness, and 20 inches from the base it is 1 inch in circumference, your terminal length will probably be 30 inches from your ponytail base.
This is kind of tricky though because if you maintain your length for a few months/years the slower growing hairs will have time to catch up to the "racers" or faster growing hairs. A lovely example of this (with photos) is Cinnamon. She is a great example of how trimming can help you grow your hair longer and longer. I want to make a point out of saying trimming won't make your hair grow faster, but it will freshen up your ends and make them stronger, and it will also give the slower growing hairs some time to catch up to the faster growing hairs. Be sure to check out her story by clicking her name, she is an inspiration.The amount of time it takes for those longest hairs to grow and shed differs from person to person based on genetics, health and environment. There are a lot of questions on this and a lot of different opinions. Some say most sets of hair will not grow for longer than 6 years of fewer than 3 before shedding. Now if your growth rate is average (.5 inches a month) that means your hairs will reach between 18 and 36 inches in length, measured from the root of the hair to the ends. The way I measure it is from my front hairline to the ends, that means my measurement is 12 inches longer than the individual hairs which start at the back of my head so they are 12 inches (24 minus 12) right now, and my hair is showing no sings that it will stop growing any time soon. This also means that if I'm average according to those estimates the hair in the back of my head hair could stop growing when it reaches 48 inches or mid thigh length on me, but if I'm not lucky it will only be 30 inches or waist length. I'm confusing myself writing all this. So what I'm saying is that according to that estimate my hair will reach terminal length somewhere between waist and mid thigh length (I'm 5'6 for reference) if I have average growth rate (.5 inches a month) and shed rate (3-6 years).
Trimming is a big topic when it comes to terminal length. Can you reach true terminal length if you trim? won't that just make your longest hairs shorter than they can be and won't it be impossible for them to reach maximum length if you shorten it? The opinions of this are varied, but in my humble opinion I believe trimming is absolutely necessary if one is trying to grow their hair as long as they can. I believe this because of a blog entry I read a while ago: White hair under the microscope. In that entry there is a photo of hair close to the roots and a photo of the ends. The ends are much thinner and come to a point. This means they are very fragile, that's why I see a lot of people talking about taking care of their ends on hair forums, that is the weakest part of your hair strand, the one that's been through the most amount of environmental damage (for the longest amount of time). Without trimming they will thin out until they break off.
People with fine hair are at the highest risk for this because their strands have less thickness to be worn off thus it happens way faster. I don't believe anyone with fine hair can grow very long hair without regular trims. If I had fine hair I would trim half of my growth every month (1/4 inches for most people), otherwise I would experience years upon years of no length gain because my ends just crumble. This also applies to damaged hair that has been permed, chemically dyed, or subjected to heat styling/blow drying.
Now you may think "hey if they break off on their own why not just let them, who needs trimming anyway?" If the hair breaks, the ends of the strand will be frayed which can easily turn into split ends, those travel up the strand (making it even thinner and more vulnerable to breakage) and entangle neighboring strands as they go causing more havoc. Trimming on the other hand gives your strands a blunt end which is thicker than it was before (because the strands thickens as you go up) and it will be less likely to break or split. Split ends are your enemy (What exactly do split ends look like?), if you have them trim them off (that's the only solution, don't believe ads, they lie, you can't glue an individual strand back together) and if you don't have them you're very very lucky.
To sum up: if you want to reach the longest lengths your hair can reach, or at least longer than you've ever been able to reach before: trim regularly, don't damage your hair, and care for your ends (oil them, protect them by keeping them in updos and generally be extra gentle with them). Alternatively henna strengthens hair so if you feel your hair needs the extra strength and you've tried everything else without results definitely give it a try (you can get different colors if you mix it with indigo, be warned using the wrong kind of henna may melt your hair off so inform yourself first).
Now which hair is the shiniest is answered in the blog entry I linked above. Black, straight hair is the shiniest. The lighter your hair color the less light it reflects and also the curlier your hair is the less light it reflects.
For more information and personal experience of different people, check out this thread:
Growing as Long as It will Go! (The Terminal Goal Thread)
Other cool links:
terminal hair length calculator
Terminal length calculator (sortof)
Estimating your terminal length
Terminal Length and the Role of Genetics in Black Hair Care
HAIR GROWTH FACTS
Trimming, Hair Growth and Terminal Length!!