The first time a private company sells shares to the public and lists on a stock exchange.
An IPO is how a company transitions from privately held to publicly traded, raising capital by selling new shares (or existing shareholders selling their stake) to public investors for the first time. Retail investors apply for shares during a fixed subscription window at a price band set by the company and its bankers; allotment is then done via lottery or pro-rata basis if the issue is oversubscribed.
The Grey Market Premium (GMP) — the unofficial, informal premium at which IPO shares trade before listing — is widely tracked as an (unreliable but popular) gauge of expected listing-day demand.