[SideburnsOfDoom]

In context of APIS, "async" communications means event notifications and message queues: Amazon SNS/SQS, Amazon Kinesis, Azure Event hub, Kafka, RabbitMQ etc. Anything where you send a message and don't wait for a response, unlike http.

[the_arun]

Minor correction to what I said. Async means we do not keep someone/something waiting for response. There are so many options on the implementation as you described.