Numbers that share identical character widths (that is, they are monospaced). Using tabular figures enables you to set columns of numbers, and have them neatly line up vertically. This is especially useful for tables, thus “tabular”. Tabular figures are usually lining, but can also be oldstyle. When the different figure sets are built-in as OpenType features, certain (older) operating systems and applications will only be able to access the default figures, most often the tabular lining figures. See Figuring Out Numerals on The FontFeed.fontshopglosary
Numerals that all have the same width. This makes it easier to set tabular matter.