Experience counters are used by a cycle of commanders in Commander 2015 to track a commander's growth in power over the course of the game.
Unlike most other counters, experience counters are placed on players, rather than on permanents.
Each legendary creature in the cycle has a triggered ability that grants its controller an experience counter, and a second ability that benefits from experience counters.
Because experience counters are put on players, they stay on you even if a commander dies. Additionally, multiple cards in this cycle could accumulate counters in tandem.
- All experience counters are identical, no matter how you got them. For example, Ezuri, Claw of Progress's last ability will count experience counters that you got from the first ability, from another ability, from proliferating, and so on. 
- Mizzix of the Izmagnus -
Legendary Creature — Goblin Wizard
Whenever you cast an instant or sorcery spell with converted mana cost greater than the number of experience counters you have, you get an experience counter.
Instant and sorcery spells you cast cost less to cast for each experience counter you have.