opening verse of a renga
Specifically the first 17-syllable (5-7-5) verse in a linked-verse sequence (renga). This is a technical term from classical poetry.
See also: 挙句 (あげく)
By extension, 発句 can refer to a standalone haiku, especially in older usage. In modern Japanese, 俳句 is the usual word for haiku.
first five-syllable line of a tanka
Rare technical use; refers to the opening 5-syllable line of a tanka (5-7-5-7-7). This usage is mostly found in classical poetry commentary.
In renga, the opening verse plays an important role in setting the seasonal tone of the whole sequence.
In Bashō's haiku, one can feel a deep insight into nature.
The first line of this tanka begins with a powerful expression that grabs the reader's attention.
短歌 is the 31-syllable poem form; 発句 can refer to its first 5-syllable line in classical contexts, but this usage is rare.
発句 is the opening stanza of a renga sequence, while 挙句 is the closing stanza, making them structural opposites in linked-verse poetry.
From 発 (starting, opening) + 句 (verse, phrase). The term originated in renga composition, where the first verse was called 発句.