The active ingredient is the substance responsible for a medicine's effect. Everything else in the tablet or liquid, such as fillers, dyes, and flavourings, is called an inactive ingredient or excipient.
Checking active ingredients is the single most useful habit for avoiding accidental double dosing, because one active ingredient can appear in many differently branded products. Both prescription labeling on DailyMed and over-the-counter Drug Facts panels list active ingredients first.