abstract
Abstraction is the process of perceiving patterns between specific instances of something in order to form a more general or universal application. All meaning, and therefore thought, is an abstraction. For example, a brick itself is not abstract, but the idea of a brick is. The brick itself exists, but the fact that we call it a brick means that it follows the pattern of "brickness"; i.e. being a solid object, usually rectangular, formed from a mold. In fact, we only learn the word "brick" by differentiating it from other objects that are not a brick, such as a block[1], rock, log, panel, sheet, etc.
Abstractions are subjective and arbitrary. Does that mean that everything is meaningless? In one sense, yes (this is the critique of Postmodernism) in that meaning is subjectively bestowed by an intellect and not a measurable, objective reality. In another sense, meaning does exist (this is the realm of Symbolism) in that abstractions are based on the perception of external realities. The categories that we used to describe the specific instances of something don't exist in tangible reality, but they describe reality and make it intelligible. Communal, or even universal agreement (intersubjectivity) imply that meaning exists externally.
Example

A block and brick are sometimes synonymous (think Lego) but a block is usually carved from a solid material. Wooden blocks are not bricks, but you might have a brick of fire starter, which is pressed from a mold. β©οΈ