The loyalty symbol (without a numeric value)
Loyalty is a characteristic of a Planeswalker, as introduced in Lorwyn.
Although mechanically similar to life, it is flavorwise not the same. Damage that would otherwise be fatal doesn't kill a planeswalker. It just discourages him or her from helping you.
From the glossary of the Comprehensive Rules (October 4, 2019—Throne of Eldraine)
- 1. Part of a card that only planeswalkers have. A planeswalker card’s loyalty is printed in its lower right corner. See rule 209, “Loyalty.”
- 2. A characteristic that only planeswalkers have. See rule 306.5.
From the Comprehensive Rules (October 4, 2019—Throne of Eldraine)
- 209. Loyalty
- 209.1. Each planeswalker card has a loyalty number printed in its lower right corner. This indicates its loyalty while it’s not on the battlefield, and it also indicates that the planeswalker enters the battlefield with that many loyalty counters on it.
- 209.2. An activated ability with a loyalty symbol in its cost is a loyalty ability. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”
- Damage dealt to a planeswalker causes that many loyalty counters to be removed from it. This is a damage result; see rule 119.3c.
- A planeswalker with loyalty 0 is put into its owner’s graveyard. This is a state-based effect; see rule 704.5i.