What is the stream of consciousness writing technique?

The stream of consciousness represents a writing technique characterized by describing the flow of thoughts that are passing through the characters' minds. With this technique, the author tends to express his/her thoughts and feelings through the characters. This writing device consists of character's unspoken thoughts and perceptions, description of their meanings, plans, or concerns addressed to themselves.

This narrative mode was used for the first time by Alexander Bain, in his book The Senses and the Intellect (1855), although it's frequently attributed to philosopher William James.

The stream of consciousness was used by many Modernist writers, including Virginia Woolf, James Joyce, Samuel Beckett, etc.