Shorthand for 'Receive Window', aka the TCP Receive Window, a tunable aspect of TCP network stacks.

In computer networking, RWIN (TCP Receive Window) is the amount of data that a computer can accept without acknowledging the sender. If sender has not received acknowledgement for the first packet it sent, it will stop and wait and if this wait exceeds a certain limit, it may even retransmit. This is how TCP achieves reliable data transfer.