n.

1. 云[C][U]
   The sky became suddenly covered with dark clouds.

   天空突然变得乌云密布。

2. （尘埃、烟等）云状物[C][（+of）]
3. （移动中的昆虫、鸟等的）一大群[C][（+of）]
   A cloud of locusts hit the farm.

   大群蝗虫袭击了那个农场。

4. 阴影；阴云[C]
5. （大理石等的）黑斑[C]

vt.

1. （云、烟等）覆盖，遮蔽
   Smog clouded the whole city.

   烟雾笼罩全城。

2. 使阴暗；使模糊；使混浊[（+up）]
   Her eyes were clouded with tears.

   泪水使她双眼变得模糊起来。

3. 使混乱
   The decision will only cloud the issue.

   这一决定只能使这一问题变得复杂起来。

4. 使忧郁，搅扰

vi.

1. 阴云密布[（+over/up）]
2. 变暗；变模糊；变混浊
   My glasses kept clouding up.

   我的眼镜老是模糊不清。

3. （脸色等）阴沉下来

n.

1. a visible mass of condensed watery vapour floating in the atmosphere, typically high above the general level of the ground.
   ▸an indistinct or billowing mass, especially of smoke or dust.
2. a state or cause of gloom or anxiety.
3. an opaque patch within a transparent substance.

v.

1. (usu. cloud over) (of the sky) become full of clouds.
2. make or become less clear or transparent.
3. make unclear or uncertain.
   ▸spoil (something).
4. (of the face or eyes) show sadness, anxiety, or anger.