A strike fighter is a fighter aircraft used to attack ground targets of high value, including ships. It differs from an attack aircraft in that the aircraft remains a fighter, able to fight other fighters, whereas an attack aircraft typically has no, or very limited, capability in this regard.
More recently the term has become increasingly blurry. The F-117 is often referred to as a strike fighter, although it has no air-to-air capability and is thus an attack plane.