Definition
Numerals with varying heights and descenders that blend harmoniously with lowercase text. The figures 0, 1, and 2 are typically x-height; 6 and 8 have ascenders; 3, 4, 5, 7, and 9 have descenders. Also known as text figures or lowercase figures. Compare with lining (titling) figures.
Source
— Robert Bringhurst — p. 327:
Numerals with varying heights and descenders that blend harmoniously with lowercase text.