Ablative armor

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

Principles

Ablative armor functions by expending the energy of destructive forces on degrading the armor itself instead of the material the armor protects.

A real-life 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.

Whether the Federation's ablative armor uses the same principles is uncertain, since Star Trek authors frequently misuse scientific terms in their scripts.