What Sin, Cos, and Tan are, your choice.
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All three are known as trigonometric functions. They are the most basic 3 of the trig functions. All 3 have inverses of Sec (secant), CSC (cosecant), and Cot (cotangent). Every single one of them can be defined entirely by any other. For example, Cot(x) = Sin(x+2π)/Sin(x) which isn’t necessarily important to know. However, what is important is that memorizing more than one is not necessary to do all the math related to all 6 functions. It is extremely helpful to memorize the big 3, being Sin (sine), Cos (cosine) and Tan (tangent) because they can make the math much much shorter and simpler.
I tend to be a minimalist. I memorize fewer equations and understand them really well, but I might be a bit slower at getting to the solution because I haven’t memorized the tips and tricks to reorganize things into other forms. Trigonometry has about a metric bajjilionton of proofs and formulas that you can memorize (half-angle formula, double-angle formula, law of sines, law of cosines, etc…) But I made it through an entire physics and math degree only remembering 2 things from trigonometry. The first is the Pythagorean theorem.
The second is the mnemonic SOH CAH TOA. Each of those 3 letter groups represents 3 things. The first letter is the trig functions Sin, Cos, and Tan. If you have a right triangle and know one other angle (besides the right angle giving it the right triangle property), then the sin, cos, or tan of that angle represents a ratio of 2 of the sides which are the other 2 letters. All angles in any triangle touch 2 sides. For the complementary angles in a right triangle (this excludes the right angle), one of those sides is always the hypotenuse (the longest side giving us the H in SOH and CAH). The other side is called adjacent (giving us the A in CAH and TOA). And since there is always a 3rd side in a triangle which does not touch that angle, this is called opposite (giving us the O in SOH and TOA).
Sow how to use the mnemonic, SOH means the Sin(angle)=Opposite/Hypotenuse. CAH means the Cos(angle)=Adjacent/Hypotenuse. And TOA means the Tan(angle)=Opposite/Adjacent. This allows you to fill in all the missing pieces of the puzzle. All you need is one side length and any other piece of information about the triangle and you can fill in every unknown (I’m excluding the right angle from this because that’s a given, so you need 3 pieces of info I guess). And in physics, every single problem can be split into an X, Y, and Z component, solved separately with very simple equations that only deal with 1 dimension, and then combined at the end.
But to recap your actual question, these functions are fed the angle (not the right angle) in a right triangle and they tell you the ratio of the lengths of 2 of the sides. This is only defined from 0 to 90 since those are the min and max allowed values for that angle in a real right triangle. But if you pretend that the direction of the sides matters (like if the triangle sits in the IVth quadrant on a graph, then it can have negative side lengths) well the. We can extend the definition to any input angle. That’s where the unit circle comes in. I’ve seen some other answers covering that already, but ask any questions you’ve got about it and I’d love to throw my hat in the ring and try explaining that too.
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