What does it mean for a system to be causal?

Related questions

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4. What is 2nd order system?
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6. What is difference between continuous and discrete signal?
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Tags

1. #function
2. #inputs
3. #depends
4. #system
5. #examples
6. #output
7. #means
8. #present
9. #future
10. #definition
11. #systems
12. #casual
13. #causal