Numerals with varying heights and alignments — some with ascenders and descenders — designed to harmonize with lowercase text. Also called old-style figures.
Definition
Text figures (also called old-style figures or non-lining figures) are numerals designed to blend with lowercase letters in running text. Unlike lining figures, which sit uniformly on the baseline and match the cap height, text figures have varying heights: some (such as 3, 4, 5, 7, and 9) descend below the baseline, while others (6 and 8) rise to ascender height. The digits 0, 1, and 2 typically align with the x-height. This variation gives text figures a quieter, less intrusive presence in prose, making them the preferred choice for scholarly and literary typography.
Source
No access — Robert Bringhurst — p. 345:
Figures designed to blend with the lowercase letters. They normally have ascending and descending forms.