In the United States Air Force a senior airman is an enlisted rank above airman first class and below staff sergeant with a pay grade of E4. The insignia for this rank is three white downward-facing chevrons centered on a blue circle with a white star in the center. A senior airman is addressed as "airman" and is abbreviated SrA.
Formerly this rank did the same strange thing as the Army and had two separate ranks for the same pay grade -- sergeant and senior airman, where senior airman was the equivalent of a specialist -- but this was abolished in 1992, leaving only senior airman.
Senior airmen are in the transition period between journeyman or worker status and NCO. They are acquiring supervisory and leadership skills through practical experience and individual study in preparation for promotion to staff sergeant.
A senior airman is an OR4 on the NATO rank scale, comparable to a Corporal of the Royal Air Force.
Airman First Class ← United States Military Ranks → Staff Sergeant
Mary Feik Award
On a completely anecdotal tangent, I met Mary Feik at my summer encampment this year -- a really classy lady who still restores aircraft in her spare time. She was one of the first women in military aircraft maintenance during World War II and the first woman to fly on test flights of new aircraft (5,000 hours logged as a flight engineer, if I remember correctly). Really a neat experience.
Anywho, a senior airman is fairly experienced as cadets go. You can trust them to know what they're doing during drill and so a lot of them are assigned as element leaders, guidon bearers, or guides. I think I've seen at least one senior airman acting as flight sergeant, but I could be hallucinating.
The only change in insignia from airman first class is the addition of another chevron -- three shiny silver stripes on the collar. The ribbon has a thin orange stripe on the outside, a thicker red stripe dead center, and is solid blue in between.
Senior airmen are addressed as "airman," and it is abbreviated C/SrA.
The jump to staff sergeant is the first "milestone" achievement a cadet encounters. While they still have to log a cadet physical fitness test (CPFT) and listen to yet another moral leadership lecture, in lieu of leadership and aerospace education tests they have to pass a comprehensive test that includes material from all previous leadership chapters and guidon drill.
Cadet Airman First Class ← Civil Air Patrol Cadet Grades → Cadet Staff Sergeant
Wikipedia: Senior Airman, Comparative military ranks
Grunt's Military: US Air Force Enlisted Insignia
Leadership: 2000 and Beyond