Unlike proportional numerals, tabular numerals (also known as tabular figures) have a fixed width. Each numeral occupies the same horizontal space, allowing them to align vertically in tables (hence the term tabular).
In metal type, numerals were often tabular by default (typically occupying the width of an en). Many contemporary fonts offer both proportional and tabular numerals which can be accessed via OpenType features. The question whether numerals are lining or non-ligning (oldstyle) is separate from whether they are proportional or tabular.