Shortfill is probably a term that you hear a lot when looking at vaping online. In 2017, the TPD laws came into effect, which meant no e-liquid could be sold containing nicotine above a 10ml bottle. You can read more about the TPD laws here.
Essentially, a shortfill is a large bottle of e-liquid that contains no nicotine. Manufacturers leave space in the bottle for you to add an unflavoured nicotine shot (‘Nic Shot’ or ‘Nic Booster’), this is where the term ‘short-fill’ comes from, as the bottle is not completely full. For example, a 50ml shortfill means you're buying 50ml of e-liquid in a 60ml bottle, with 10ml of space for one 18mg nicotine shot. When this 18mg 10ml shot of nicotine is diluted with 50ml of 0mg e-liquid, it will make the whole liquid 3mg.
You can calculate how many Nic Shots you need for your shortfills with our Nic Shot Calculator here, to get to your desired strength.