In Japanese, the kateikei 仮定形 is one of the six basic inflectable forms of verbs and adjectives.
The term kateikei is used with modern Japanese. The term izenkei 已然形 is used with classical Japanese. The kateikei evolved from the izenkei. Although they're similar, their functions are different.
For example, in nomeba 飲めば, the nome~ 飲め～ part is the kateikei or the izenkei. In modern Japanese, it's the kateikei, and nomeba means "if [you] drink [it]." In classical Japanese, it's the izenkei, and nomeba means "because [you] drink [it]."