Loyalty

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. Flavorwise, damage that would otherwise be fatal doesn't "kill" a planeswalker, it just discourages them from helping you and causes them to leave (sent to graveyard).

Rules

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Loyalty

1. Part of a card that only planeswalker cards 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 (June 7, 2024—Modern Horizons 3)

• 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 (see rule 306.5b).
  • 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.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 120.3c Damage dealt to a planeswalker causes that many loyalty counters to be removed from that planeswalker.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 704.5i If a planeswalker has loyalty 0, it’s put into its owner’s graveyard.

Rulings

• 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.