Nicotine Shots also referred to as Nic Shots or Nicotine Boosters, are small bottles of flavourless e-liquid that are added to zero-nicotine Shortfill e-liquids to give you a larger quantity of nicotine-containing juice. Shortfill bottles leave enough empty space for you to mix a nicotine shot straight into the bottle; this is often more cost-effective than buying individual 10ml bottles of juice. 

An 18mg nicotine shot (10ml) added to a 50ml short fill bottle of zero-nicotine e-liquid would give you 60ml of e-liquid, now boosted to 3mg (0.3%) nicotine.