Rank is the term used in Diablo III for a spell or skill's level. This term should (hopefully) reduce some of the confusion caused by the word "level" being used for so many things. Player levels, spell levels, "levels" = areas of the dungeon, etc.
For example, the Wizard skill Thunderstruck is listed as follows:
Thunderstruck
- Passive Skill
- Description: Increases the critical hit chance of all lightning spells by X%. Critical hits from lightning damage stun targets for 2 seconds.
- Max Rank: 15
- Rank 1: Increases critical hit chance of all lightning spells by 5%.
- Rank 2: Increases critical hit chance of all lightning spells by 10%.
- Rank 3: Increases critical hit chance of all lightning spells by 15%.