An illegal play in which an offensive player attempts to impede the progress of a downfield defender in order to allow a teammate to get open for a pass. Usually, this entails a pair of receivers running a crossing pattern past one another with one receiver designated to make enough contact with the defender to allow the other receiver to come open. If the official determines the contact to be intentional or egregious, he can throw a flag for offensive pass interference.
The pick play is a common play throughout football, and is a technique that requires a team to design plays that can cause a pick without the flag being thrown for pass interference. Often, the picking player will have to manage to impede the defender's progress just enough to slow him down while not making the contact obvious enough to draw the flag. Most crossing patterns are designed to put players in positions that can create traffic that causes a pick situation without the obvious pick occurring.