• (erection) In medicine, the swelling of the penis with blood, causing it to become firm.
  NCI Dictionary of Cancer Terms
  U.S. National Cancer Institute, 2021
• (penis erection) Physiological response of penile erectile tissue engorging with blood due to male sexual arousal.
  CRISP Thesaurus
  National Institutes of Health, 2006
• The hardening, enlarging and rising of the penis which often occurs in the sexually aroused male and enables sexual intercourse. Achieved by increased inflow of blood into the vessels of erectile tissue, and decreased outflow.
  Gene Ontology Dictionary
  Gene Ontology Consortium, 2021
• The state of the PENIS when the erectile tissue becomes filled or swollen (tumid) with BLOOD and causes the penis to become rigid and elevated. It is a complex process involving CENTRAL NERVOUS SYSTEM; PERIPHERAL NERVOUS SYSTEMS; HORMONES; SMOOTH MUSCLES; and vascular functions.
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2021
• Erection, enlargement, hardening, and elevation of the male reproductive organ, the penis. Internally, the penis has three long masses of cylindrical tissue, known as erectile tissue, that are bound together by fibrous tissue. The two identical areas running along the sides of the penis are termed...
  Encyclopedia Britannica
  Encyclopedia Britannica, Inc., 2020