Summon Cost

Summon Cost is the Mana Cost you pay when you summon a creature.

Details
It is written in text as 召喚する時支払うコスト (shōkan suru toki shiharau kosuto) meaning "Cost to pay when summoning".

It doesn't actually alter the mana cost of cards in the battle zone, only during the summon itself. This is important to know for Cost Based Removal.