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)
In renga, the opening verse plays an important role in setting the seasonal tone of the whole sequence.
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 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.
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.