position 18 in a countable series of things
"She celebrated her eighteenth birthday last week, marking her as the final guest at the party since she was the youngest invited."
The person or thing in the eighteenth position.
"She was thrilled to find out she had come in as the eighteenth runner across the finish line."
coming next after the seventeenth in position
"She was the eighteenth person to arrive at the party, standing right behind her best friend."
The ordinal form of the number eighteen.
"The eighteenth item on our grocery list was surprisingly expensive."