Illegal procedure

In the National Football League, an illegal procedure can be used to refer to one or any number of a set of penalties. It is not itself a particular penalty, but an encompassing term which can describe several infractions.

Types of illegal procedure
In many cases, most notably in the National Football League, this type of penalty has even been given its own signals. Examples of this include: However, certain penalties still retain the "illegal procedure" signal. Examples of this are:
 * Illegal shift
 * Illegal motion
 * Illegal forward pass
 * Illegal touching of a forward pass
 * Ineligible receiver downfield
 * Illegal substitution
 * False start
 * Illegal formation
 * Kickoff or safety kick out of bounds
 * Player voluntarily going out of bounds and returning to the field of play on a punt

Usage by the referee
Most penalties that are considered to fall under the category of "illegal procedure" are not stated as such when announced by the referee, as it may be considered insufficient explanation, and require further justification anyway. However, referees do often preface the less common penalties of "kickoff or safety kick out of bounds," and "player voluntarily going out of bounds" with, "illegal procedure," likely to avoid possible confusion caused by the signal with multiple meanings.

Usage at different levels
It is used similarly to the professional level at the high school level, but this term is not referenced at all in the collegiate rule book.