Life

Life is a concept of points. Players start a game of Magic with 20 life points. One of the ways of winning a game is to get your opponent to 0 life.

Loss of life

Loss of life can occur from unblocked creatures, direct damage from spells. There are spells which can prevent loss of life (i.e. Samite Healer

Life gain

Life gain directly opposes the game’s primary win condition. The earliest "life gain" spells include Healing Salve, Lifetap, Stream of Life. Stream of Life is notable for being the first gain X life spell. There were also 5 artifacts printed which would give you 1 life each time a spell of the appropriate color was cast if you paid 1 (i.e. Ivory Cup).

The biggest life gain spell, not X is the card Heroe's Reunion (7) from Invasions.