As new therapies for psoriasis are developed, it becomes increasingly important to have reproducible definitions for examining outcomes of therapeutic interventions. Herein, key definitions for use in clinical trials are presented that represent the consensus opinion of the Medical Advisory Board of the National Psoriasis Foundation. It is acknowledged that these definitions are mathematical constructs designed to model and quantify, for clinical study purposes, clinical concepts that are well recognized. Our objective is to provide greater uniformity and clarity in reporting results of clinical trials for patients with psoriasis. These definitions also give dermatologists a basis for comparing outcomes of different therapies.