Wiktionary
alt. A music, poetry, or comedy stage platform with open invitation, that is, where anyone is invited to perform. n. A music, poetry, or comedy stage platform with open invitation, that is, where anyone is invited to perform.
Wikipedia
An open mic or open mike (derived from open microphone) is a live show where audience members may perform at the microphone. Usually, the performers sign up in advance for a time slot with the host or master of ceremonies. These events are typically focused on performance arts like poetry and the spoken word, music, and comedy.
Usage examples of "open mic".
He knew his ex-girlfriend would be here in about an hour to play at that ridiculous open mic she always went to but still, it would probably be pretty cool to stick around and hear her new songs except for the fact that his friends told him she recently cut off her hair and bleached it blonde to piss him off and it did piss him off because he had been trying to convince her to do that the entire time they were dating and she was doing it now just to spite him.
Then one night while she was free-styling on an open mic, Marlene had walked up, gave her a business card, and a supposed record deal—.