a idea or thought but not existing

A form of art that allows plenty of freedom and flexibility for both the artist and audience to their own interpretation of the art's meaning and value. It is to not simply portray or replicate something of its realistic form but give insights to its value in a different dimension of thinking.

Of or pertaining to the formal aspect of art, emphasizing lines, colors, generalized or geometrical forms, etc...

Abstract in art: Abstract art is the subjective artwork with no defined process, strategy and/or technique label; it allows the viewer to interpret its meaning with his or her objective view of life in that specific art work; allowing artists to explore and represent a wide variety of pieces, from canvas and space to sculpture and more with no predetermined standard for good or bad art.

Basically it can be used sarcastically as another way of saying your art piece sucks..

Or it can be said as a compliment; it just depends on how you take it.

1. When someone thinks out of the box.

When somebody doesn't want you to take something seriously, they tell you to think abstract. So concrete thinking would be solid, or tight thinking, and abstract thinking would be liquid, or loose thinking (flexible).