1. 巨大;庞大;重要(性)
    If you talk about the magnitude of something, you are talking about its great size, scale, or importance.

    e.g. An operation of this magnitude is going to be difficult...
           这么大的手术实施起来十分困难。
    e.g. These are issues of great magnitude...
           这些问题极为重要。

2. 星等(指星的亮度);震级;(爆炸的)强度
    Magnitude is used in stating the size or extent of something such as a star, earthquake, or explosion.

    e.g. ...the 1.2 magnitude star Fomalhaut...
           星等为1.2 的北落师门星
    e.g. The San Francisco earthquake of 1906 had a magnitude of 8.3.
           1906年旧金山地震的震级为8.3级。

3. 程度;重要性
    You can use order of magnitude when you are giving an approximate idea of the amount or importance of something.

    e.g. America and Russia do not face a problem of the same order of magnitude as Japan.
           美俄两国所面临问题的重要性与日本的不可同日而语。

4. 数量级
    In mathematics, if one amount is an order of magnitude larger than another, it is ten times larger than the other. If it is two orders of magnitude larger, it is a hundred times larger.

    e.g. The time delay would be smaller by eight orders of magnitude.
           如果按8个数量级来看，延时量会显得小一些。