A spell type is any subtype used for the further classification of instant and sorcery cards.
Other than creature types spell types don't have to appear on all sorceries and instants. Instead they are only introduced for mechanical purposes.
From the glossary of the Comprehensive Rules (July 3, 2020—Core Set 2021)
- Spell Type
- A subtype that’s correlated to the instant card type and the sorcery card type. See rule 304, “Instants,” and rule 307, “Sorceries.” See rule 205.3k for the list of spell types.
From the Comprehensive Rules (July 3, 2020—Core Set 2021)
- 205.3k Instants and sorceries share their lists of subtypes; these subtypes are called spell types. The spell types are Adventure, Arcane, and Trap.
Adventure only appears on instant or sorcery spells attached to creature cards.
Arcane spells are the main target of the Splice mechanic.
Trap is only used on instants. They have an alternative cost should specific events occur. Usually, traps have abilities that undo whatever has made their alternative cost possible.