What is Irrevocable License?

Legal Definition
One in the exercise of which the licensee has made improvements or invested capital. See 87 Cal. 126, 22 Am. St. Rep. 234, 11 L. R. A. 134, 25 Pac. 268, one connected with an interest or grant. See 22 Am. St. Rep. 238, note.
-- Ballentine's Law Dictionary