Once a transaction is committed to the blockchain is there a way to let an entity outside the blockchain (i.e. An entity that is not a node in the blockchain) know that a particular transaction has been committed?
event definitions and
emit steps are the go-to contract design step to be courtious to external clients using event listeners. Similar to subscribing to a feed and they can listen to specific contracts and topics, apply filters and so on.
An entity that is not a node
This is sort of a challenge. If they don't have their own then they will rely on something that does to inform them. Somebody (reliable) has to be listening to Ethereum.
You can set up your own caching server to catch interesting events and pass them on, or rely on a third-party API to do it. Something like: https://blog.infura.io/faster-logs-and-events-e43e2fa13773/
Hope it helps.