seven-and-five-syllable meter
A traditional Japanese poetic meter alternating lines of 7 and 5 syllables, common in waka, haiku, and other classical forms.
Waka are often composed in a seven-and-five-syllable meter.
From 七 (seven) + 五 (five) + 調 (meter, tone). The term directly describes the 7-5 syllable alternation.