Initial Public Offering. The very first time a private company enters the public stock market and sells its shares openly. In other words, a debut of any private company into the public stock exchange market. Many IPO's are sold in pre-market value before they enter the actual public offering (usually sold between banks and institutions in early stages). For many, IPOs mean you could own a hypothetically cheaper option of a bigger company - grab a promising company for the entry price, then re-sell it for higher value a week later.
IPOs also pose the contrary dangers of being over-valued at the launch date and may fall off favour after the actual market hype disappear. Some notorious examples of IPOs: Goldman Sachs (1999), China Life Insurance (2003), AT&T (2000), Alibaba (2014), Snapchat (2017), etc.