Wiktionary(0.00 / 0 votes)Rate this definition: 1  kern(Noun) A light-armed foot soldier of the ancient militia of Ireland and Scotland; in archaic contexts often used as a term of contempt. 2  kern(Noun) any part of a letter which extends into the space used by another letter. kern(Verb) To adjust the horizontal space between selected pairs of letters (characters or glyphs); to perform such adjustments to a portion of text, according to preset rules.