Define Camper.

I am relatively new to the Multiplayer scene, and recently I was starting out on certain Half-life MOD, and I was on the defending team. There were two entrances to the area we were defending. Entrance number one was a partially closed garage-type door, which was really obvious and which everyone defended, while number two was a ventilation tunnel to the upper area of this place we were defending. The other team would frequently flank us and catch us in a crossfire, but no one tried to guard that vent. I took it into my mind to do so. I took a large caliber rifle and sat at one end of the vent, opposite where the enemy would drop down, and whenever they tried to flank us, I would shoot them. After a few times one of the other team started calling me a camper as well as a few other insults. I thought a camper was someone who guarded an item, like a healthpack or weapon or something. I thought I was keeping the enemy from flanking us. Sorry for such a long post but could somebody clear up this camper thing for me