Initial public offering
The first sale of shares by a private company to the public.
More broadly, the term sometimes refers to offerings of shares to selected institutional investors (also known as a placing) leading to the company's shares being listed on a public market.
More broadly still, any corporate activity leading to a company's securities becoming traded in the public markets.
IPOs are often issued by smaller, younger companies seeking the capital to expand, but can also be used by large privately owned companies looking to become publicly traded.