could

1. (auxiliary verb, defective) To know how to; to be able to.

   She can speak English, French, and German.   I can play football.   Can you remember your fifth birthday?

2. (modal auxiliary verb, defective) May; to be permitted or enabled to.

   You can go outside and play when you're finished with your homework.   Can I use your pen?

3. (modal auxiliary verb, defective) To have the potential to; be possible.

   Animals can experience emotions.

4. (auxiliary verb, defective) Used with verbs of perception.

   Can you hear that?.

5. To know.

noun

Something that could happen, or could be the case, under different circumstances; a potentiality.