Combat arms as those branches of service with the mission of making direct contact with enemy forces and defeating them with systems effects and maneuver. "Effects" replaces the older term "fire", since some of the most important means of defeating modern militaries are "nonkinetic" such as electronic warfare. "Contact" must be interpreted in an era of long-range systems with both kinetic and nonkinetic effects.
Included in the definition are infantry, units of armored fighting vehicles, artillery, Air, artillery and missile defense, aviation under the control of land commanders, combat engineers, Air, artillery and missile defense, and special operations forces. Different nations, at different times, have had different names for these functions.
Not all these functions existed in older militaries. The term cavalry goes back to time immemorial, but its basic functions of reconnaissance and shock attack have now been given to armored fighting vehicles, light ground vehicles and aircraft. Armored fighting vehicles have variously been termed "Tank Corps", "Armor Branch", "Heavy Forces", etc.
"Special operations" and "special operations forces" again have meanings specific to times and places.