Level
Definition
When applied to a character, the term "level" can refer to a number of things.
Security Level / Threat Level
A character's Security Level (for Heroes) or Threat Level (for Villains) reflects the number of times the character has visited a Trainer to level up...plus one, since newly-created characters are Security/Threat Level 1 without ever seeing a trainer.
Security/Threat Level is used to limit a character's participation in Task Forces, Strike Forces, and Trials, and his entry into zones with a level requirement.
Experience Level
A character's Experience Level is the maximum Security/Threat Level that the character has the experience to reach, regardless of whether they have actually trained enough times to reach it.
This level is compared against Enhancement levels to determine their relative potency. It is also the default value for a character's Combat Level (see below).
Combat Level / Effective Level
A character's Combat Level, also called their Effective Level, is equal to their Experience Level when solo. On a team, characters who have been sidekicked up will be one level below the team's level, and characters who have been examplared down will be at the team's level.
A character's Combat Level determines their base health and the base strength of all their powers. It, along with the Combat Level of the character's enemies, affects their chances of hitting and getting hit.
Challenge Level
Another term for notoriety, the setting that controls the difficulty of missions and task forces relative to your security/threat level.