position 13 in a countable series of things
"She stood as the thirteenth person to sign the guest book at the wedding reception."
The person or thing in the thirteenth position.
"The thirteenth runner across the finish line was declared the winner of that particular heat."
coming next after the twelfth in position
"She was the thirteenth player to score a goal during the match, stepping up right after her teammate who had just made it twelve."
The ordinal form of the number thirteen.
"The thirteenth guest arrived just as the cake was being cut."
Thirteenth comes from Middle English and Old English, where it was originally formed by combining the word for thirteen with an ordinal suffix. This construction indicates that it is the number following twelve in a sequence.