Period

Definition:

Of, belonging to, or representing a certain historical age or time: a period piece; period furniture.
-Dictionary.com

Period comes up when dicussing metal armor and garb. To look "period" basically means not wearing steel armor and sweat pants. If it didn't exist at the point in time your character is from, then it isn't period. Some people get really picky about this, and others could care less.

See Also:

• Book of War