Invulnerability

The health bar usually has this golden frame to indicate the invulnerability status.

Invulnerability is a status effect that reduces all damage taken to 0 (except the internal raw damage dealt by Nexus Obelisk and Sion's Glory in Death's self-inflicted damage) and skips their damage events entirely. It is the only form of damage reduction that affects true damage. All units are invulnerable at the end of a match.

Life steal, spell vamp, drain, and pre-mitigation effects (such as Death Mark) will have no effect against units in the invulnerability state. An invulnerable champion with Thornmail will still reflect damage.

Invulnerability may occasionally be used to refer to:

Effects that prevent falling below a certain health amount, which are Kindred's Lamb's Respite and Tryndamere's Undying Rage. All damage taken in excess of the unit's health after the threshold is ignored, not negated / reduced. Effects that trigger from damage (e.g. life steal and shields being depleted) will still apply, and as a consequence, this is not invulnerability. Note that Lamb's Respite grants invulnerability to units that reach the threshold.
Fiora's Riposte, Karthus' Death Defied, Urgot's Fear Beyond Death, and Zac's Cell Division which similarly ignore damage but do not prevent effects that trigger from damage (e.g. life steal applies and shields will absorb damage before the prevention). Note that invulnerability makes the unit ignore damage before those specific events can take place. Thus, those abilities do not grant invulnerability.
Untargetability, such as Sanguine Pool or Featherstorm, which is explicitly not considered invulnerability because units that are untargetable are only invalid targets to targeted effects that would normally apply damage, meaning already-applied effects that deal damage (like damage over time) will still affect them. Note that stasis grants both status effects.
Spell shields, which are capable of completely mitigating a hostile ability. However, this is not guaranteed, as some spells or components of an ability cannot be blocked and will apply their effects as normal.
Dodging, blocking, and blinding effects (also known as parries), which are separate mechanics that only prevent the instance of damage dealt by basic attacks and on-hit effects (with a few exceptional cases).