Canon is a concept concerning the truthfulness of events portrayed as being part of (or related to) a narrative. Using the traditional perspective of what canon means, all parts of the narrative nominally seen as canon are considered entirely true with relation to each other within the narrative. On the other hand, non-canon events or depictions, such as speculative derivative works (fanfiction) and the like, are not considered to be true or "real" from within the main "canonical" body of work. Speculative derivative works may use elements, settings or characters from a narrative, but are not considered canonical with respect to the narrative proper.
As a collective noun, a "canon" denotes a collection of distinct works or parts of a work that are together considered to be canonical with respect to each other.
In the Homestuck universe, canon is a tangible aspect of reality that can be interacted with from within the narrative under certain situations. Andrew Hussie, the webcomic's author, also exists as a character within the canon, and can directly intercede in events that occur personally, while it is also acknowledged that he is the one writing the story. More directly, the "treasure" Juju, functioning as a literal "plot hole", is a gateway with which to interact with the canon, as it functions as a physical manifestation of a "narrative breach" that must be filled. John Egbert fills the role of this breach by interacting with the Treasure, granting him the ability to wield Retcon mechanics, powers that let him directly interact with the canonical narrative of Homestuck.
"Noncanon" in Act 7 and beyond 
In Act 7, the claiming of the Ultimate Reward through departure to Universe C allows the main cast of the Homestuck canon to leave canon entirely, and exist "out of canon". It is this property that allows for them to escape Lord English, whose ultimate downfall lies in his existence being bound (or stuck) in a self-perpetuating loop of existence within the Homestuck canon, that begins and ends with his rise to power (and defeat). These two events may be circumstantially simultaneous.
The "out of canon" state that the primary characters find themselves in is a state distinct from non-canonicity, wherein the events that they are living may indeed be "true", but are dependent on the events that occur within the canon. This fact leads to the main crisis of the Homestuck Epilogues; in order for canon to proceed in the manner that will allow these events happening "out of canon" to be "real", then John, who alone can still have a role to play in Homestuck's canon due to his retention of his retcon abilities, must use them to travel back into the Homestuck canon and defeat Lord English once and for all. Within canon, these may be the events depicted Caliborn's Masterpiece, although this is still speculation. If John fails to do this, then canon will not be able to sustain the "out of canon" events on Earth C, as depicted in the Credits, as real. That reality would be rendered null and void, a process that Rose Lalonde refers to as "dissipation".
The uncanny similarity between the universe C entry door on the Victory platform and the juju that releases the god tier kids to fight Lord English likely reflects how they are, respectively, doors to exit and re-enter canon.
Canon, outside canon, and non-canon
List of Homestuck universe works by canonicity
- Homestuck: The main definition of "canon."
- Hiveswap and Hauntswitch: Canon.
- The Homestuck Epilogues and Homestuck^2: Beyond Canon: The definition of dubiously canon. The stories often explore the concept of canonicalization.
- Paradox Space Comics: Dubiously canon. Some stories are suggested to be able to "fit" into canon, but no stories are referenced in canon and can largely be viewed as non-canon.
- Hiveswap Friendship Simulator and Pesterquest: Dubiously canon. While certain elements and characters use their canonical counterparts, it is assumed the timelines explored are different from Homestuck and Hiveswap.
- MSPA Snapchat: Non-canon. Confirmed by the team. Snapchats appearing in the Credits, of course, are still canon.
- Namco High: Non-canon. Possibly due to the game being publicly unavailable.