A subject is a being who has a unique consciousness and/or unique personal experiences, or an entity that has a relationship with another entity that exists outside of itself (called an “object“). A subject is an observer and an object is a thing observed. This concept is especially important in continental philosophy, where ‘the Subject’ is a central term in debates over human autonomy and the nature of the self.
The sharp distinction between subject and object corresponds to the distinction, in the philosophy of René Descartes, between thought and extension. Descartes believed that thought (subjectivity) was the essence of the mind, and that extension (the occupation of space) was the essence of matter.