Null session

A null session in Homestuck is any session of the game that is doomed to failure.

While this refers to any session that fails to create a universe, is also seems to refer to sessions where the players create a universe but are still unable to win. Both Karkat Vantas and Aradia Megido HS 4467 (call their) HS 4509 (own session) a null session after they were prevented from obtaining the ultimate reward, even though they successfully created a universe and killed the Black King.

According to Rose Lalonde, null sessions HS 6601 (are actually very common). As Skaia is "aware" of its own fate, null sessions are actually designed to fail from the beginning. The Scratch can offer a second chance of sorts by resetting the universe, though the new session that results may also be a null session.

The Session Glitch is hinted to create a null session.

Void session
A HS 6601 (void) HS 6602 (session) is a specific type of null session where none of the sprites are prototyped pre-entry. On the one hand, this prevents the Underlings and monarchs from gathering any significant powers from prototypings. On the other hand, such a session cannot be won as the Battlefield will not reach it's final form, preventing the creation of a new universe.

In a void session, both Prospit and Derse lack prototyping towers, as Skaia is already aware there will be no pre-entry prototypings.

Examples
Every session seen or heard about in Homestuck is some kind of null session.

Although the precise circumstances are unknown, the players of pre-Scratch Alternia were likely in a null session due to the Session Glitch.

The Troll players of post-Scratch Alternia, as previously stated, were successful in creating a universe. However, it seems their session became null after Jack Noir prevented them from entering the newly created universe. It is implied that and  had deliberately engineered this result through their subtle machinations and corruption of troll society.

The pre-Scratch Homestuck Kids also had a null session. It seems that this is the result of Karkat giving their universe cancer by failing to properly breed the Genesis Frog in his session. This cancer manifested as Jack Noir, who ruined the session by killing the Black Queen and Black King before The Reckoning, and as The Tumor, which created the Green Sun.

The post-Scratch human session is actually a void session. Jane Crocker is unable to prototype anything before entering her session, and it is implied a similar fate will happen to the other players.