In philosophy and logic (especially metaphysics), a property is a characteristic of an object; for example, a red object is said to have the property of redness. The property may be considered a form of object in its own right, able to possess other properties. A property, however, differs from individual objects in that it may be instantiated, and often in more than one object. It differs from the logical and mathematical concept of class by not having any concept of extensionality, and from the philosophical concept of class in that a property is considered to be distinct from the objects which possess it. Understanding how different individual entities (or particulars) can in some sense have some of the same properties is the basis of the problem of universals.

A property is any member of a class of entities that are capable of being attributed to objects. Terms similar to property include predicable, attribute, quality, feature, characteristic, type, exemplifiable, predicate, and intensional entity.

Generally speaking, an object is said to exemplify, instantiate, bear, have or possess a property if the property can be truly predicated of the object. The collection of objects that possess a property is called the extension of the property. Properties are said to characterize or inhere in objects that possess them. Followers of Alexius Meinong assert the existence of two kinds of predication: existent objects exemplify properties, while nonexistent objects are said to exemplify, satisfy, immanently contain or be consubstantiated by properties that are actually possessed and are said to encode, be determined by, be consociated with or be constituted by properties that are merely ascribed to objects. For example, since Pegasus is merely mythical, Pegasus encodes the property of being a horse, but Pegasus exemplifies the property of being a character of Greek mythology as well. Edward Jonathan Lowe even treated instantiation, characterization and exemplification as three separate kinds of predication.

Broadly construed, examples of properties include redness, the property of being two, the property of being nonexistent, the property of being identical to Socrates, the property of being a desk, the property of being a property, the property of being both round and square, and the property of being heterological. Some philosophers refuse to treat existence as a property, and Peter van Inwagen suggested that one should deny the existence of certain "properties" so as to avoid paradoxes such as Russell's paradox and Grelling–Nelson paradox, though such moves remain controversial.

In modern analytic philosophy there are several debates about the fundamental nature of properties. These center around questions such as: Are properties universals or particulars? Are properties real? Are they categorical or dispositional? Are properties physical or mental?

At least since Plato, properties are viewed by numerous philosophers as universals, which are typically capable of being instantiated by different objects. Philosophers opposing this view regard properties as particulars, namely tropes.

A realist about properties asserts that properties have genuine, mind-independent existence. One way to spell this out is in terms of exact, repeatable, instantiations known as universals. The other realist position asserts that properties are particulars (tropes), which are unique instantiations in individual objects that merely resemble one another to various degrees. Transcendent realism, proposed by Plato and favored by Bertrand Russell, asserts that properties exist even if uninstantiated. Immanent realism, defended by Aristotle and David Malet Armstrong, contends that properties exist only if instantiated.

The anti-realist position, often referred to as nominalism claims that properties are names we attach to particulars. The properties themselves have no existence.