The definition for combustion is "Act of burning." Used in a sentence: Campers must use fire with great caution to prevent combustion of the parched brush.

The definition for commemorate is "To mark by a ceremony." Used in a sentence: Each year, the Franklins commemorate their anniversary by visiting the site of their first date.

The definition for elusive is "Hard to grasp." Used in a sentence: The words to the song are elusive, as the singer tends to mumble.

The definition for epistolary is "Contained or carried on in letters." Used in a sentence: The shoebox filled with letters was the ony remains of Ming's epistolary friendship with Emile.

The definition for retroactive is "effective as of a prior time or condition." Used in a sentence: The retroactive law made even past infractions punishable.