The protocol data unit at OSI layer four (transport layer) is a called a segment. A segment consists of a layer four header encapsulating an application payload.
One easy way to visualize the meaning of segment is to think of network application data as a very lengthy stream. In order to packetize this data for transport, first this data stream must be divided into manageable pieces. The process of dividing the application data stream into pieces is called segmentation. The results of this division are called segments.
Note that segments by themselves are not sufficient for network transmission. The only addressing provided by the layer four header is source and destination port number. Port numbers identify the nature of the application payload, but they say nothing about the receiving host's network location, nor do they support any particular method of physical transport. To push segments over the network, further encapsulations at layers three and two will be required.