The shares which are not listed on the formal stock exchanges are referred to as unlisted shares/ stocks. For instance, JIO has unlisted shares, OLA has it as well. Similarly, many companies are yet to go public as they do not comply with the requirements for being listed on a formal stock exchange.
Unlisted shares are riskier than listed shares as their liquidity is limited since it is not listed. They are less transparent but with more stable valuations. So, if you can pick an unlisted share that has all the potential to get listed and the company has growth potential, your returns can be amplified enormously from that share.