Sin (sine), cos (cosine) and tan (tangent) are the core three functions used in trigonometry and are based on a right-angled triangle.

“Opposite” is opposite to angle θ “Adjacent” is adjacent to angle θ “Hypotenuse” is the remaining side

They are each a measurement of ratio for the sides of a right angled triangle:

$\sin{θ} = \frac{opposite}{hypotenuse}$

$\cos{θ} = \frac{adjacent}{hypotenuse}$

$\tan{θ} = \frac{opposite}{adjacnet}$

because the calculation is a measure of ratio, size does not matter - only the angle is able to change the ratio.

These ratio operations can be remembered using the acronym SohCahToa:

Sine → opposite / hypotenuse Cosine → adjacent / hypotenuse Tangent → opposite / adjacent

Research Sources

1.1. Introduction to Sin, Cos and Tan