A task is single instance of the execution of a transaction.
This word, task, has a specific meaning in CICS. When CICS receives a request to run a transaction, it starts a new task that is associated with this one instance of the execution of the transaction type. That is, a CICS task is one execution of a transaction, with its own private set of data, usually on behalf of a specific user. You can also consider a task as a thread. Tasks are dispatched by CICS according to their priority and readiness. When the transaction completes, the task is terminated.