The custodian is covered in full in the entry Bank Custodian, which describes the services custodian banks provide, how they are regulated and the largest firms in the sector. This page gives a short definition and explains the broader use of the word.
A custodian is a financial institution that holds and safeguards assets, such as shares, bonds, cash or, increasingly, crypto assets, on behalf of clients. Beyond keeping assets safe, custodians settle trades, collect dividends and interest, handle corporate actions, process tax reclaims and report holdings. Investment funds are usually required to appoint an independent fund custodian so that managers cannot simply take investors' money, and client assets held by a custodian are generally kept separate from its own, which protects them if the custodian fails. In crypto, regulated firms that hold private keys for institutions are described as a digital asset custodian.
Most large custodians are banks, which is why the topic is covered under "bank custodian", but trust companies, broker-dealers and specialist crypto firms can also act as custodians. The service itself is described under custody service.