73 in ham radio:
73 is a code that means "best regards" or "best wishes." It's used at the end of a communication to signal a friendly farewell. The term comes from the Phillips Code, a shorthand numerical code developed in the 19th century for telegraphic communications. "73" has been adopted by amateur radio operators worldwide as a traditional sign-off.
Many abbreviations you may be familiar with today originated in the Philips Code.