1. 有根据的;正当的;合理的
    A valid argument, comment, or idea is based on sensible reasoning.

    e.g. They put forward many valid reasons for not exporting...
           他们提出了很多不出口的正当理由。
    e.g. It is valid to consider memory the oldest mental skill, from which all others derive...
           我们有理由认为记忆是最原始的心智技能，所有其他技能都在此基础上衍生出来。

2. 有意义的;有效的
    Something that is valid is important or serious enough to make it worth saying or doing.

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    e.g. Most designers share the unspoken belief that fashion is a valid form of visual art.
           大多数设计者都认同这样一种不言而明的理念，即时尚是视觉艺术的一种重要形式。

3. （票证等）有效的
    If a ticket or other document is valid, it can be used and will be accepted by people in authority.

    e.g. For foreign holidays you will need a valid passport...
           出国度假需持有效护照。
    e.g. All tickets are valid for two months.
           所有的票在两个月内有效。

4. see also: validity