realis form; perfective form
In classical Japanese grammar, the 已然形 is a verb/adjective form used to express realis (already realized) conditions, often followed by ば to mean 'since' or 'because'. In modern Japanese, it corresponds to the 仮定形 (hypothetical form) when followed by ば, but the classical 已然形＋ば indicates a reason, not a hypothesis.
See also: 仮定形 (かていけい)
In classical grammar, 'realis form + ば' expresses a cause or reason.
The '書け' in '書けば' is the hypothetical form in modern Japanese, but it is considered the realis form in classical Japanese.
The 未然形 is the irrealis form in classical Japanese, used for negation, passive, causative, etc. It contrasts with the 已然形, which indicates something already realized.
From 已然 (izen, 'already so') + 形 (kei, 'form'). The term originates from traditional Japanese grammar (国学) analysis of classical Japanese.