1. 病人;病弱者;伤残者
    An invalid is someone who needs to be cared for because they have an illness or disability.

    e.g. I hate being treated as an invalid.
           我讨厌被人当作伤病号对待。

2. 无效的;作废的
    If an action, procedure, or document is invalid, it cannot be accepted, because it breaks the law or some official rule.

invalid的反义词

    e.g. The trial was stopped and the results declared invalid...
           审判被中止,判决结果被宣布无效。
    e.g. We cannot accept liability if you are refused entry because of invalid documents.
           如果你因证件无效被拒入内,我们不承担责任。

3. (论据、结论)站不住脚的,错误的
    An invalid argument or conclusion is wrong because it is based on a mistake.

    e.g. We think that those arguments are rendered invalid by the hard facts on the ground.
           我们认为那些论点当场即被确凿的事实驳倒了。