n.

1. 集中[C][U]
   There is a concentration of people in big cities.

   大城市人口集中。

2. 专心，专注[U]
   Concentration is essential if you want to do a good job.

   如果你想把事情做好，专心是必要的。

3. 浓缩；浓度[U][S1]

n.

1. the action or power of concentrating.
2. a close gathering of people or things.
3. the relative amount of a particular substance contained within a solution or mixture.