Also known as syllabic verse: a verse in which each line has a fixed number of syllables. In some languages such as Japanese this is sufficient to create metrical form, as in the case of the haiku, but English metre counts beats rather than syllables, and so isosyllabic verse in English has functioned (in the work of poets like Marianne Moore and W. H. Auden) as a modernist alternative to metre, a way of providing more structure to the writer in the process of writing even if the result is, as Thom Gunn remarked in abandoning the technique, “indistinguishable from free verse”.
Groves, Peter Lewis. "Isosyllabic verse". The Literary Encyclopedia. First published 04 June 2007
[https://www.litencyc.com/php/stopics.php?rec=true&UID=5515, accessed 21 March 2019.]