The waterproof jacket you throw on before heading out on a drizzly gray day is a raincoat. If you're traveling to India during monsoon season, be sure to pack your raincoat!
Simply put, a raincoat is a coat that protects you from getting drenched in the rain. You might also call it a "slicker" or a "rain jacket." Some raincoats are truly waterproof, while others are merely water resistant — these will keep you dry for a while but are less useful if you're walking miles in a downpour. The original "modern" raincoat was invented in 1824 by a Scottish chemist who layered fabric and rubber to create a new waterproof material.