What is "equal decrement of life"

Decrement \Dec"re*ment\, n. [L. decrementum, fr. decrescere. See Decrease.]

1. The state of becoming gradually less; decrease; diminution; waste; loss.

   Twit me with the decrements of my pendants.
   --Ford.

   Rocks, mountains, and the other elevations of the earth suffer a continual decrement.
   --Woodward.

2. The quantity lost by gradual diminution or waste; -- opposed to increment.

3. (Crystallog.) A name given by Ha["u]y to the successive diminution of the layers of molecules, applied to the faces of the primitive form, by which he supposed the secondary forms to be produced.

4. (Math.) The quantity by which a variable is diminished. Equal decrement of life.

   1. The decrease of life in a group of persons in which the assumed law of mortality is such that of a given large number of persons, all being now of the same age, an equal number shall die each consecutive year.

   2. The decrease of life in a group of persons in which the assumed law of mortality is such that the ratio of those dying in a year to those living through the year is constant, being independent of the age of the persons.