Catalyst Monster

Catalyst monsters are monsters with "Catalyst" listed on the Type/Ability line. They are required for the Genesis Summon of a Genesis Monster. Effect, Pendulum, and Genesis Monsters themselves can all be Catalyst monsters.

To perform a Genesis Summon, the player sends monsters they control (excluding a Tuner monster) out of play whose total amount of Cores is exactly equal to that of a Genesis Monster in their Extra Deck, then they Genesis Summon that Genesis Monster. Usually, Genesis Summons require exactly 1 Catalyst monster, but some Genesis Monsters allow or require multiple Genesis monsters.

A number of support cards specifically deal with Catalyst monsters, which can further aid in setting up a Genesis Summon.

Some Genesis Monsters require a specific catalyst monster to be used ("Archangel Michael"), or a Catalyst of a specific Type ("White Wings Defender"), Attribute ("WIP"), or archetype ("WIP"), etc.

Level
Catalyst monsters are often set in Level, usually ranging from Level 1 to 12. Higher level catalysts need tributes to summon them but also contain more benefits or a wider range of summon-able Genesis monsters.

Effects
All Catalyst monsters have effects that assist in performing a Genesis Summon, such as by Special Summoning another monster, Special Summoning themselves when a condition is met ("Lady of the Lake", "Fishborg Archer"), or managing a type for "Genesis" Summon.

Etymology
As implied by the name, several Catalyst monsters are based off of grand items or symbols, ones that are seen as the "Catalyst" for a war or massive event.