deviation. 1  the act of deviating. 2  departure from a standard or norm. 3  Statistics. the difference between one of a set of values and some fixed value, usually the mean of the set. 4  Navigation. the error of a magnetic compass, as that of a ship, on a given heading as a result of local magnetism. 5  Optics.
The definition of an invitation is a request for a person's attendance at an event. An example of an invitation is a printed card asking family and friends to come to the sender's wedding. 1  an inviting to come somewhere or do something. 2  the message or note used in inviting.  enticement or allurement.
The invited or optional means that male guests are invited to wear tuxes and women may wear evening gowns or cocktail dresses -- what you would wear at a black-tie affair -- but you don't have to. It's fine for men to wear nice dark suits and for women to wear dresses that aren't necessarily Oscar-night material.