Ham is an informal and time-honored name for an amateur radio operator. Amateur radio and ham radio are synonymous, as are amateur and ham in the context of radio.
What a strange moniker for such a noble practitioner! How did we get to be called hams?
Well, the absolute truth is lost to history and there is a fair bit of disagreement and some urban legend out there. But we know that poor telegraph operators were called hams (for ham-fisted) in the late 1800s.
The first radio (wireless) operations involved Morse code communication so a number of professional telegraphers got caught up in early radio. Amateurs were right along side them, experimenting with their own private wireless gear. Because radio communications in the early days involved broad-band spark transmitters, there was no tight frequency control and plenty of interference.
Obviously the professional wireless operators were frustrated at amateur interference and no doubt they complained about it over the air for all to hear. It’s quite likely that they referred to the amateurs in a pejorative manner as “hams”, either from the telegraphy context or possibly derived from the term ham as a bad stage actor.
Apparently the amateurs didn’t understand that they were being insulted, or they just didn’t care. Somewhere along the way they adopted the term ham in a favorable light (perhaps a counter-culture attitude?) and used it proudly.
However it really happened, the name ham has been applied to amateur radio operators for around a hundred years.
Three informative references on the origins of the term ham are found here:
The History of Amateur Radio (An comprehensive and interesting read by itself)
ARRL Ham Radio History (click under What is a Ham?)
We can never know for sure how the name ham came about for amateur radio operators. Study up and form your own opinion; it’s a fun topic to explore.