Definition:Triangle (Geometry)/Right-Angled/Adjacent

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In a right-angled triangle, for a given non-right angled vertex, the adjacent side which is not the hypotenuse is referred to as the adjacent.

In the above figure:

the adjacent to vertex $A$ is side $c$
the adjacent to vertex $C$ is side $a$.

Also known as

Some older sources, particularly in the context of schoolbooks, refer to the adjacent as the base.

This arises from the habit of orienting the triangle (as is common in such sources) so that its opposite is a vertical line, at the side of the figure (as above).

However, confusion can arise when the triangle is positioned differently, and the angle in question is not at the bottom.