en passant

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adverb In passing; by the way; incidentally.
• adverb Used in reference to a move in chess in which a pawn that has just completed an initial advance to its fourth rank is captured by an opponent pawn as if it had only moved to its third rank.

from The Century Dictionary.

• While passing; by the way: often used as introductory to an incidental remark or a sudden disconnected thought.

from the GNU version of the Collaborative International Dictionary of English.

• (Chess) In passing; in the course of any procedure

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adverb in passing; incidentally; by the way
• noun chess A move in which a pawn captures an opposing pawn on the same rank immediately after the latter moved two squares from its initial position as if it had captured it while the latter was passing through.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adverb incidentally; in the course of doing something else
• noun (chess) a chess pawn that is moved two squares can be captured by an opponent's pawn commanding the square that was passed