对于一个直角三角形，直角边（源自希腊字Κάθετος，复数Κάθετοι）；而英文的复数是catheti，是取自拉丁文cathetus的复数，常用的解释是"leg" ，即一个直角三角形中，形成90度的两条相邻的边。而余下的一条边，与直角相对，称为斜边。又因为毕氏定理在中文常被称为勾股定理或勾股弦定理，故在中文里，直角边常常被称为股，此乃延续中国古代的称呼。"leg"这种表达方式在多数情况之下并不常用，一般都用直角边或一个更迂回的说法：＂位于直角的一边＂。当提及到斜边，直角边一般被理解为余下的两边。

In a right triangle, a cathetus (originally from the Greek word Κάθετος; plural: catheti), commonly known as a leg, is either of the sides that are adjacent to the right angle. It is occasionally called a "side about the right angle". The side opposite the right angle is the hypotenuse. In the context of the hypotenuse, the catheti are sometimes referred to simply as "the other two sides".