Also written as "q.s." Short for quantum satis, one of the less common Latin abbreviations. It means "a sufficient quantity". Mainly used when describing preparation of a chemical or biological/pharmaceutical reagent: to "add an ingredient to a QS of 500 mL" means, essentially, to keep adding something until the product volume is 500 mL.
As a technical writer, I find a lot of older documents that have instructions like "QS with X to a final volume of y mL". This term, like "aliquot", should never be used as a verb, at least according to Dorland's Illustrated Medical Dictionary.
Note that this is not to be confused with "q. suff.", short for quantum sufficit, which means "as much as suffices". Q.suff. does not need to describe a set amount.