tangential coma

tangential coma

[tan′jen·chəl ′kō·mə]
(optics)
For a lens that displays coma, the length of a tangent from the vertex of the patch of light formed in the focal plane by rays from an off-axis point to the comatic circle of rays from this point that pass near the edge of the lens.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.