Merriam-Webster Collegiate Dictionary - lifetime

 I. noun  Date: 13th century  1.  a. the duration of the existence of a living being (as a person or an animal) or a thing (as a star or a subatomic particle)  b. life 12  2. an amount accumulated or experienced in a ~ a ~ of regrets  II. adjective  Date: 1834  1. lifelong  2. of long duration or continuance ~ legislation  3. measured or achieved over the span of a career a baseball player's ~ batting average