Meaning of trapdoor function,

trapdoor' func"tion

Pronunciation: [key]
— Math. Math.
  1. a function defined from data by means of a mathematical procedure in such a way that it is easy to obtain the function when the data are known, but when the procedure and data are not known it becomes very difficult to determine the original data: used in cryptography, where the data are the characters of the plain text, or message, and the trapdoor function is the cryptogram.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.