Titer: The concentration of a substance in a fluid, such as the concentration of an antibody in blood. The titer reflects the amount of the substance in the fluid. Sometimes also spelled titre. SLIDESHOW Could I Be Allergic? Discover Your Allergy Triggers See Slideshow From Allergies Resources Featured Centers Good and Bad Foods for PsoriasisVideo: Getting Personal on Life With MS Health Solutions From Our Sponsors Shot-Free MS Treatment Your Child and COVID-19