When trying to analyze an animal's body language, each signal must be taken in context. Other body language signals should be analyzed as well to form as complete as possible view of the animal's intent or emotional state. You can generally characterize signs in 3 categories: go, stop and yield. 

​Generally, the crouched position is one of defensiveness, self protection or fear. It is a yield sign. That means that interaction with a cat in this position could end well or poorly. At minimum, the cat is uncomfortable with the situation. The crouched, lying position involves folding the feet underneath the body and often the tail is curled around the body. 

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This cat is scared. Her feet are underneath her, her tail is tucked around her body and her pupils are dilated.

This cat has his tail behind him instead of next to him. His pupil size is normal but his front legs are tucked underneath him. He isn’t scared, but is likely annoyed by the person who sticking a camera in his face.

This cat is hiding. She is at the veterinarian’s office. Her pupils are dilated, her tail is around her body and she has her front feet tucked under.

Here is the same cat as on the left when he is relaxed.