Immunity (Su)
Source: Advanced Player's Guide, p. 62
An eidolon’s body becomes extremely resilient to one energy type, gaining immunity to that type. Pick one energy type: acid, cold, electricity, fire, or sonic. The eidolon gains immunity to that energy type. This evolution can be selected more than once. Its effects do not stack. Each time it applies to a different energy type. The summoner must be at least 7th level before selecting this evolution.