The origin of the military title colonel goes all the way back to the 16th century. King Ferdinand of Spain organized his army in 1505, dividing it into "colunelas," which translates into "columns" in English. Each colunelas was composed of more than 1,000 men all lead by the cabo de colunela or head of the column. These colenulas divisions were then copied by the French and English armies who even retained the colunela title for their regimental heads. Over the centuries, the pronunciation of colunela evolved into the current "colonel." Addressing a military officer that is a retired colonel has set guidelines laid out by protocol.
A colonel should always be addressed by his earned rank in social conversation. Whether he is retired or on active duty, proper protocol calls for the use of his appropriate title. If you are addressing a letter or invitation to a retired colonel, use only his name and rank and not the branch of service in which he served or retired rank.
When a retired military officer is involved in commercial enterprises, you should follow the Department of Defense's directive which bans the use of a title. For example, if a colonel who retired from the U.S. Air Force went to work for Ford Motor Co., she would be addressed as Ms., Miss or Mrs. at work instead of her military rank. This is different from how you would address her in a social setting where she would be referred to as a colonel.
In written correspondence, there are additional protocol in addressing a retired colonel. When sending official correspondence to a retired officer, use either the designation Ret. or Retired. First, address the envelope using the officer's rank and name followed by a comma. Next, write the service branch followed by another comma andthen the Ret. or Retired designation. For example, a letter could be addressed to Col. John Smith, USMC, Retired, or to Col. John Smith, USMC, Ret. Either form of the word "retired" is considered acceptable on written correspondence. When writing social and official correspondence also mark all letters with the branch of the service from which the colonel retired. If the servicewoman retired from a different branch of service than the one they entered, the branch the retired from is the one noted. For example, if she served in the Marines but retired while a member of the Marine Corps Reserves, correspondence should be addressed to Col. Jane Smith, USMCR, Retired.