Thanx....it's an acronym that's been around as long as I've been in Broadcast Frank....over 30 years....see below(much quicker than I could 'splain it)...
A public service announcement (PSA) or public service ad, are messages in the public interest disseminated by the media without charge, with the objective of raising awareness, changing public attitudes and behavior towards a social issue....and it is required by the FCC to satisfy one of the requirements of operating in the public interest....