Sigil is the catch-all term for a symbol, rune, or glyph used to channel magic. Most often, a sigil is used to refer to the name of an Archon in this context. Spell casting works on the principle that Archons create new magical phenomena, and mages then imitate these powers - it is the Archon's sigil that a mage must invoke to channel that Archon's power.
Proper spell casting uses not only the core sigil (for example, a ball of fire would need to start with the sigil of the Archon of Fire) but sigils for shape (a 'bolt' sigil in the fireball example) and for various modifications. Skilled mages have been known to uses sigils that prolong, extend, echo, or intensify their spells.
In-game, Spells are made up of at least two sigils: a Core Sigil and an Expression Sigil. Accent and Enhancement Sigils give the spell additional properties. The more sigils a spell contains, the higher the Lore required to cast it.
Each spell can only have one Enhancement Sigil, while Accent Sigils of different categories can be added (and their effects stack).
Stacking rules : Expression Sigils also determine what beneficial spells can stack with one another. For example, the Gift of the Golem spell (Guarded Form expression) will stack with Titan's Touch (Focused Intent expression) and Spectral Blur (Focused Intent expression) will stack with Mirror Image (Guarded Form expression). But Titan's Touch will not stack with Spectral Blur as they both use the expression Focused Intent.
All learnt Sigils will carry over into New Game+.
Lists of sigils