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. 

A paw lift in cats means something similar to what it means in dogs which is: anticipation. When a cat lifts her paw, something is about to happen. The cat may think that you are going to give her a treat or pet her. She may be about to swat you. How will you know? Look at the rest of the body. Is the tail thumping? Are the ears back? Stop in your tracks! The swat is coming! Is she looking at you with a soft flagging tail, a soft face and her ears forward? Is she softly meowing to you? She is most likely soliciting your attention. 

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This cat has lifted her paw in response to a person approaching. Her tail is in a neutral position and her ears are forward. Her pupils are not dilated. In this case, she lifted her paw most likely because she was uncertain about what the approach would mean. When approached quietly from the side, this cat sat down and solicited attention from the stranger.

This cat is raising her paw and her pupils are slightly dilated. She is aroused from play and is reaching to swat.