1. 一生；终生
    A lifetime is the length of time that someone is alive.

    e.g. During my lifetime I haven't got around to much travelling.
           我一生中没怎么去旅行过。
    e.g. ...a trust fund to be administered throughout his wife's lifetime.
           为他妻子终生管理的信托基金

2. （事物的）存在期，使用期
    The lifetime of a particular thing is the period of time that it lasts.

    e.g. ...the lifetime of a parliament.
           议会的届期
    e.g. ...a satellite's lifetime.
           卫星的使用期

3. 终生难遇的；千载难逢的
    If you describe something as the chance or experience of a lifetime, you are emphasizing that it is the best or most important chance or experience that you are ever likely to have.

    e.g. This could be not just the trip of a lifetime but the experience of a lifetime.
           这可能不仅是一趟终生难遇的旅行，也将是一次终生难遇的体验。