A firearm- Usually a handgun or revolver. This term came from the 30's mafia/gangster times. When continually fired, the barrel would get very warm- hence the name heater.
"Before we go outtonight, do you got your heater?"
Don't mess with her, she's got a heater.
A heater should be interjected into any form of speech. You can be on a heater. You can be heatering. You can be sending heaters. You can be a heater. When you are on a heater, you will know, and anybody that isn't on a heater won't understand. If you spend enough time around the same people though, they will start to get it and join the clan of Heater Wan Kenobis. When you go to the mountain to shred, you will instantly be able to tell which people are on a heater because they will be sending the biggest heaters. When you go to the bar, you will be searching for some heaters to bring home with you. When you get in a grueling beer pong match, the heaters will start heating up before their first toss. Being on a heater isn't a fad, it's a way of life and the only way to reach true enlightenment.