Definition
A trigonometric function of an angle in a right triangle, defined as the ratio of the length of the side adjacent to the angle to the length of the hypotenuse. Values range from -1 to +1.
Plain English
A number you get from an angle. For any angle in a right-angled triangle, cosine tells you how the side next to that angle compares in length to the longest side (the hypotenuse).
Context Anchor
Seen in aviation maintenance math when working with angles, slanted distances, forces, or component calculations.
Derivation
From the Latin 'complementi sinus,' meaning 'sine of the complement.' The cosine of an angle equals the sine of its complementary angle (the angle that, added to it, makes 90 degrees). The 'co-' prefix in cosine, cotangent, and cosecant always signals this complementary relationship.
Why Pilots Care
Trigonometric functions appear throughout maintenance and engineering work -- calculating force components, analyzing structural loads, and working with vectors. A technician who can read 'cos' on a formula or calculator can follow the math without getting stuck.
Intuition Check
cos is not a unit and not an angle by itself. It is a calculation based on an angle that gives a ratio between two sides of a right triangle.
Example Sentence 1
To find the horizontal component of the force, the technician multiplied the total force by the cos of the angle.
Example Sentence 2
In vector analysis, cos helps resolve the load into horizontal and vertical parts.