Ablative armor

Ablative armor is a protective material that functions by expending the energy of destructive forces on degrading the armor itself instead of whatever the armor protects.

A good example is the heat-shield used to protect space capsules and missiles returning to Earth from outside the atmosphere. An ablative coating on the re-entry vehicle vaporizes at high temperatures, expending the heat from friction with the atmosphere to keep the vehicle itself from growing too hot and melting.

Ablative Armor in Star Trek

Ablative armor is seen on newer Federation starships such as the Defiant-class, Prometheus-class, and Sovereign-class. It can protect a ship from multiple hits from Dominion weapons or Klingon disruptors before failing. Ablative armor allows a ship to stay in combat even after its shields have failed or briefly lower its shields to use transporters during combat.

Whether the Federation's ablative armor uses the same principles as real-life ablative armor is uncertain, since Star Trek authors frequently misuse scientific terms in their scripts. However, from visual effects it would seem that ablative armor dissipates energy of beam weapon hitting it over much larger area than it would affect normally, and combines that with ablation effect in order to prevent catastrophic hull breaches by reducing or cancelling damage to main hull and spreading it over large area.