Deaths within a population recorded on a per-cohort basis.
Life tables are a demographic description of what number as well as fraction of a population, of a given age, remain alive at the start of a span (e.g., year) versus at the end of a span. The resulting information can then be used to calculate including on a per age basis, determine , and also generate survivorship curves.
Note that by keeping track of the number of present per cohort, one also can infer within a population. Such data is complicated, however, should immigration from or emigration to the focus population occur.