A subject is a being who has a unique consciousness and unique 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.
In the modern continental tradition, which may plausibly be said to date from Descartes, debates over the nature of the Subject play a role comparable to debates over personhood within the distinct Anglo-American tradition of analytical philosophy.