An assist is the action of helping an allied champion kill someone. You can either score an assist by hitting (without killing) the champion in the last 10 seconds before their death, or by contributing passively during this period of time.
When a champion is killed, the killer gets a certain amount of gold, while 50% of that amount is divided up among everyone who got an assist on it.
Secondary System
- Champions also gain assists on kills if they:
- Applied any type of crowd control, except
Blind, 
Reveal, or 
attack speed slow, whether or not it damaged the enemy champion. - Debuffed an enemy, even if that enemy was immune to the damage.
- Supporting a champion that contributed to the kill (e.g. healing, shielding or buffing an allied champion).
- Hit an enemy champion with
Anivia's Crystallize, Trundle's Pillar of Ice or Veigar's Event Horizon.
- Champions will not gain assists if they:
- Only use Blind, Reveal, or attack speed slow, without also contributing in any of the above ways.
Triggering effects with assists
- Champion abilities, items and runes that trigger an effect with an assist can be seen on the takedown page.