There's no universally accepted meaning/definition to the words "response to treatment" or "remission" within the scientific literature you'd see on pubmed. Rather each study defines it in their own way. It makes it rather hard to compare remission-rates, response-rates and other results across a lot of different studies. As far as guides our doctors use for management of our illnesses, there are definitions broadly accepted "symptomatic remission" (based purely on patient symptoms) and "endoscopic/deep/quiescent remission" which is clean biopsies/colonoscopy with no sign of active inflammation.
There's some attempt to standardize things using a Mayo Score as an example. https://www.ncbi.nlm.nih.gov/pmc/articles/pmc2597552/