What does it mean for a side to be 'adjacent' in trigonometry?

In trigonometry, a side is considered 'adjacent' when it is next to the working angle, commonly denoted as theta (θ). This side forms part of the triangle with the angle in question and is crucial for defining the relationships between the sides of the triangle when using trigonometric functions like sine, cosine, and tangent.

The adjacent side, combined with the opposite side and the hypotenuse, allows for the application of these functions to determine various properties of the triangle. In particular, the cosine function of an angle is defined as the length of the adjacent side divided by the length of the hypotenuse, highlighting the importance of identifying which side is adjacent for accurate calculation and understanding of trigonometric concepts.