In philosophy, a subject is a being that exercises agency, undergoes conscious experiences, and is situated in relation to other things that exist outside itself; thus, a subject is any individual, person, or observer. An object is any of the things observed or experienced by a subject, which may even include other beings (thus, from their own points of view: other subjects).

A simple common differentiation for subject and object is: an observer versus a thing that is observed. In certain cases involving personhood, subjects and objects can be considered interchangeable where each label is applied only from one or the other point of view. Subjects and objects are related to the philosophical distinction between subjectivity and objectivity: the existence of knowledge, ideas, or information either dependent upon a subject (subjectivity) or independent from any subject (objectivity).

In English the word object is derived from the Latin objectus (p.p. of obicere) with the meaning "to throw, or put before or against", from ob-, "against", and the root jacere, "to throw". Some other related English words include objectify (to reify), objective (a future reference), and objection (an expression of protest). Subject uses the same root, but with the prefix sub-, meaning "under".

Broadly construed, the word object names a maximally general category, whose members are eligible for being referred to, quantified over and thought of. Terms similar to the broad notion of object include thing, being, entity, item, existent, term, unit, and individual.

In ordinary language, one is inclined to call only a material object "object". In certain contexts, it may be socially inappropriate to apply the word object to animate beings, especially to human beings, while the words entity and being are more acceptable.

Some authors use object in contrast to property; that is to say, an object is an entity that is not a property. Objects differ from properties in that objects cannot be referred to by predicates. Some philosophers include abstract objects as counting as objects, while others do not. Terms similar to such usage of object include substance, individual, and particular.

There are two definitions of object. The first definition holds that an object is an entity that fails to experience and that is not conscious. The second definition holds that an object is an entity experienced. The second definition differs from the first one in that the second definition allows for a subject to be an object at the same time.

One approach to defining an object is in terms of its properties and relations. Descriptions of all bodies, minds, and persons must be in terms of their properties and relations. For example, it seems that the only way to describe an apple is by describing its properties and how it is related to other things, such as its shape, size, composition, color, temperature, etc., while its relations may include "on the table", "in the room" and "being bigger than other apples". Metaphysical frameworks also differ in whether they consider objects existing independently of their properties and, if so, in what way. The notion of an object must address two problems: the change problems and the problems of substances. Two leading theories about objecthood are substance theory, wherein substances (objects) are distinct from their properties, and bundle theory, wherein objects are no more than bundles of their properties.