A grey market is an unofficial market where securities are bought and sold outside a formal exchange. In trading it usually refers to pre-IPO shares or instruments that change hands before they are officially listed.
A grey market runs over the counter on private agreements between buyer and seller. Prices come from supply, demand, and how confident participants are that the security will actually be issued or listed.
A grey market is the unofficial counterpart to regulated exchange trading, and it carries higher risk. Settlement can depend on whether the official issuance goes ahead, so you can face counterparty risk, pricing uncertainty, or a cancelled trade that a regulated exchange would not expose you to.
A company plans to list its shares through an IPO at USD 20 per share.
Before the official listing, the shares trade in the grey market at USD 25.
You buy at USD 25 because the price points to strong demand, but the trade stays unofficial until the shares formally list and settlement conditions are met.