In Quake 3 multiplayer, Invisibility is a powerup that makes the user almost invisible for 30 seconds. The player becomes transparent with some shimmering light effects around him/her. While it doesn't make player truly invisible, it makes him/her very difficult to see unless very close. The Invisibility power-up respawns after 3 minutes. The similar powerup appears in Quake 4.
Invisibility should be used to sneak up on targets. Don't run in front of them and fire, because they still can see you when you get close. Instead, fire from a distance or from behind them. If somebody sees you, it shouldn't be difficult to get away. Just stick to a path and they'll lose you.
It'll be difficult to tell if an enemy has invisibility, so you can't prepare yourself against it. Instead, just be aware of your surroundings. If somebody does attack, run away! It's not worth getting killed all because you wanted to chase a target that you couldn't see from medium or long range. Take it out on somebody else.
When you have invisibility, shotguns and other slow-firing weapons are your best friend. If they can see your muzzle flash, they know where you are, and invisibility becomes pointless, beam type weapons like the Lightning Gun will give your position away while firing.
Railgun is a high risk, high reward weapon for invisibility users, remember to move away after killing one enemy, because the enemies will trace the rail and get to your sniping location, unless you are really good at taking out multiple enemies quickly, they will most likely to get a retaliation kill.
Some basic console commands like mipmap might be able to make player spot invisibility wielders easily. So, mind that you're not fully invisible and not everyone are hackers.
Invisibility's usefulness depends to a great extent on the player(s) you are playing against. Some people spot invisible players easily, while others cannot see them, even if they are standing right in front of them. Watch your opponents carefully to see how observant they are. Use this knowledge to your advantage.
Hit and run is crucial to invisibility users, even you are not detected, the enemy will know someone is nearby once they hear gunshots or see their teammates killed by unseen source.
If you are playing against bots they won't be able to see you, Unless you fire.