1. 液体;流质
    A fluid is a liquid.

    e.g. The blood vessels may leak fluid, which distorts vision...
           血管可能会渗漏，造成视力模糊。
    e.g. Make sure that you drink plenty of fluids.
           务必保证足够的水分摄入。

2. (动作、线条或设计)流畅优美的，优雅自然的
    Fluid movements or lines or designs are smooth and graceful.

    e.g. The forehand stroke should be fluid and well balanced.
           正手击球应该保证动作流畅和平稳。
    e.g. ...long, fluid dresses.
           线条优雅流畅的长裙

3. 不稳定的;不确定的
    A situation that is fluid is unstable and is likely to change often.

    e.g. The situation is extremely fluid and it can be changing from day to day.
           情况极不稳定，每天都有可能变化。