See also: Manual/Techniques
A rocketjump is a technique to reach high or far away places by sacrificing some health. A rocket jump is sometimes referred to as an RJ.
Single Rocketjump Edit
A rocket jump requires timing. To perform a rocketjump hit the ground with a rocket below you just after (in practice you hit the fire and jump key simultaneously) pressing the jump key. It requires lot of training to do it reliably every time.
Instead of a rocket you can also use a grenade. Shoot the grenade, stand on it and jump as it explodes. Requires even more skill than a standard rocket jump.
Examples: Rocketjumps can be used to reach the rail platforms on oasago2 and get to the quad from the button level on dm4ish (the bots sometimes do it on that level)
Double Rocketjump Edit
Instead of one rocket use two. On standard OA maps it is almost impossible to do it with two rockets (it's difficult to jump down from high places, that give you the opportunity to fire a second shot while still falling, and reach the ground when both of them hit the floor). But there are some easier variations: e.g. you you can combine a grenade and standard rocketjump to get extreme height (in this case, it is also called a grenade-rocket-jump). Or you may ask another player to hit you from below when you are mid-jump, giving you more push. Or you may do a rocket jump on the floor first, then a second one against a wall. A double rocketjump takes a lot of health.
There are no maps in standard OA where double rocketjumps are very useful but it can be used to explore maps.
Triple and more Edit
It is possible to perform even more spectacular rocketjumps than these but mostly for fun.
Elimination, Last Man Standing Edit
In the Elimination, CTF Elimination and Last Man Standing game modes, self damage (\elimination_selfdamage) is off by default. This allows you to perform rocket jumps more frequently without fear of losing health. See also Elimination#CVARs.
G_elimination, All rockets Edit
When using g_elimination special game option, the damage to yourself is controlled by \elimination_selfdamage variable. This allows to do rocket jumps without hurting yourself in any gametype, and can be very useful together with all rockets mode (since with "all rockets" mode alone, you would die after a couple of RJs). See also Configuration examples/All rockets mode with rocket jumps.