Is it safe for my cat to drink rainwater or puddle water outside?

Occasionally, no. A single lick from a clean puddle is unlikely to cause harm, but standing outdoor water can carry runoff from fertilizer, antifreeze, bird droppings or parasite eggs, none of which are things you can see by looking at it. An indoor cat who gets brief supervised outdoor or balcony time and takes an occasional sip is a low-risk situation. A cat who is deliberately choosing puddle water over her indoor bowl is usually telling you something about the indoor water, not that she has developed a taste for rain.