Per 《新华词典》: 七言 means a seven-character poetic line and, by extension, 七言诗 — poetry in seven-syllable lines. Rose to dominance in the Tang dynasty and remains the most familiar classical line length to modern readers.
memory
Count syllables: 五 = 5, 七 = 7. Famous opener 床前明月光 = five characters = 五言; 床前明月光彩亮 (made up) would be seven = 七言. The number is literally how many characters per line.
