Mana regeneration refers to the rate at which a player's mana pool replenishes over time. With enough (MR) "mana regen" you can substitute it instead of having a skill like meditation or focus to regenerate it for you. There is a cap of 21 mana regeneration in Ultima Online currently. Mana regeneration is a property every character needs if they consume mana. There are also spells and abilities in Ultima Online that can directly affect mana regeneration. For example, there are spells that can increase a player's mana regeneration for a limited time, or there may be items that grant a player a passive mana regeneration bonus.