(Or "host-host layer") The middle layer in the OSI seven layer model. The transport layer determines how to use the network layer to provide a virtual error-free, point to point connection so that host A can send messages to host B and they will arrive un-corrupted and in the correct order. It establishes and dissolves connections between hosts. It is used by the session layer.
An example transport layer protocol is Transmission Control Protocol (TCP).
OSI documents: ITU Rec. X.214 (ISO 8072), ITU Rec. X.224 (ISO 8073).