Regeneration rate (regen) is the rate at which your armor (shields), health and power recharge.
Armor regen[]
Armor regen is indicated on the armor tooltips, and can be enhanced with armor and weapon modifiers. Note that certain types of armor regen faster than others, the fastest being Hazmat Armor and Bio Armor and the slowest being Mech Armor and Stealth Armor. Note that if your armor is entirely depleted it will stop regenerating until you leave combat for several seconds.
The Engineer signature ability Base Wave dramatically increases armor regen for the Engineer and any players nearby.
Health and power regen[]
Health and power regen can be seen on the attributes window (default key: P). It can be increased primarily by putting attribute points into Spirit, and secondarily by putting points into Mind. Armor and weapon modifiers can also increase this value. Wearing or standing near a player wearing Bio Armor will increase your health regen, and wearing or standing near a player wearing Mech Armor will increase your power regen.
The Medic signature ability Regeneration Wave dramatically increases health and power regen for the Medic and any players nearby.
Out of combat regen[]
After 10 seconds without taking damage you are considered to be out of combat and your armor, health and power regenerate much faster. This can be done even when enemies are chasing you, by standing behind buildings or rocks to obscure yourself. The extra regen will stop when you take damage again.
If you are significantly higher level than your enemies, you will stay in the "Out of combat" mode and will regenerate at an extraordinary rate, even while you are being attacked. This usually occurs with enemies that no longer give you experience.
You can easily stand in front of a whole CP worth of enemies with no armor equipped and not die.
A Boss enemy may attack you with a skill that does enough damage to put you into "In combat" regeneration mode even though you no longer receive experience from killing him.