Dab Nails, Concentrate Nails, or Banger Nailsare the glass Piece that you drop your concentrate in to do traditional "dabs". They are usually made from quartz to withstand the higher temperatures needed from torching and vaporize your oils. Bangers come in all different styles like classic buckets, thermal or reactor core styles, or the more newly popular terp slurpers - each style boasting its own unique benefits aiming to provide you with the highest quality hits.
18mm female is not one of the most common joint size/gender for bangers so please be sure to double check to make sure this is the correct size that you need! You will typically see these on larger bongs or rigs.