What is the meaning of "quiddity"?

"Quiddity" is a term for the essence of an object, literally its "whatness" or "what it is." It comes from the Middle English “quidite”, from the Medieval Latin “quidditas” meaning “essence.” This in turn came from the simple from Latin word “quid” meaning “what.” The idea derives from a similar term in Aristotle's Greek “to ti ên einai” or "the what it was to be (a given thing)".

Quiddity describes properties that a particular thing or person shares with others of its kind. The question "what ('quid') is it?" asks for a general description by way of commonality. This is quiddity or "whatness" (i.e., its "what it is"). Quiddity was often contrasted by the scholastic philosophers with the “haecceity” or "thisness" of an item, which was supposed to be a positive characteristic of an individual that caused it to be this individual, and no other individual.

As an example, consider the question, What is a "tree"? We can only see specific trees in the world around us - the category "tree" which includes all trees is a classification in our minds; but this classification is not empirical, and not observable. The quiddity of a tree is the collection of characteristics which make it a tree. This is sometimes referred to as "treeness."