An inventory is an itemized list of everything you have. So you better take inventory of what’s in the fridge before going grocery shopping. Otherwise you'll end up with more jars of mustard that you don't need.
The noun form of inventory developed in the early 15th century from the Middle French inventaire, meaning “detailed list of goods.” It's most commonly used in retail: a shop's inventory is everything that they have in stock, ready to sell. You can also use inventory as a verb when you want to describe the act of listing out the merchandise or supplies you have on hand.