Twelfth means number twelve in a sequence or list. If you're the twelfth person in line at the water fountain, eleven people are ahead of you. December is the twelfth month of the year, coming after November and before January starts the cycle again.

The word can also mean one of twelve equal parts, like when you slice a pizza into twelve pieces and take one slice. That slice is one twelfth of the whole pizza, written as 1/12 in math. If twelve friends equally split the cost of a gift, each person pays one twelfth of the total price.

You'll see twelfth written as 12th, with the “th” showing it's an ordinal number (describing position) rather than a cardinal number like “twelve” (describing quantity). The spelling is a bit tricky because of the cluster of consonants, which can trip up your tongue if you say it too quickly.