A style/color of hair (usually in a bob with bangs) which appears on first glance to be a wig. The person's hairline is not visible, except for perhaps a very messy (or overly accurate) part in the middle or side. Generally of one-dimensional color, i.e. black, dark brown (no highlights), white-blonde, etc.
Audrey Tautou's hair is almost, but not quite, wig hair in "Amelie." The back of her hair is too short. Louise Brook's short bob is a great example, as is Catherine Zeta-Jones's hair in the movie "Chicago."