勃起 (bó qǐ) is a formal, clinical term meaning 'erection' — the physiological process in which the penis becomes firm and enlarged due to increased blood flow. The character 勃 (bó) conveys vigor, suddenness, or flourishing (as in 勃发 'to burst forth'), while 起 (qǐ) means 'to rise' or 'to arise'. Together, they literally suggest 'a sudden rising' or 'vigorous rising', reflecting the rapid, involuntary nature of the physiological response. This term is used primarily in medical, scientific, or health-education contexts — for example, in discussions of sexual health, urology, or pharmacology.

It is neutral in tone but highly technical; it is not used in casual, humorous, or slang contexts. Learners should avoid using it in everyday conversation unless discussing health matters with professionals. Unlike colloquial or euphemistic terms, 勃起 carries no emotional or judgmental connotation — it describes a normal, involuntary bodily function, similar to how we might refer to 'knee-jerk reflex' or 'pupillary response' in English.