In TDM technology a 64kbps DSO (one voice call). To telephone companies, all POTS lines are considered trunks, even ones that terminate to end user telephone sets. They assume all lines are trunks because they do not know what the user has connected on the other end (i.e. multiline telephones or key systems).
To telephone/telecom/networking gear equipment manufacturers, a trunk is a connection resource between switches that can be utilized by any end user station, or network appliance. So in this aspect, a trunk could be a DS0 between central office switches or a PBX. It could also be a Gigabit Ethernet connection between two Ethernet switches, where they are referred to as VLAN trunks.
In mobile wireless it is a little more complicated because you add codecs into the equation, and in many networks it also adds VoIP into the equation as well. So, your trunks then become G.711 (which is a packetized form of the TDM DS0 at 64kbps) or G.729, (which is 8kbps). For the VoIP part, packet header overhead needs to be added, which makes a G.711 packet voice stream (or packet voice Trunk) require 80kbps for G.711 and 12kbps for G.729. Though in the voice world G.729 is generally used, the IP Telephony manufacturers are now making their gear G.729 compatable as well.
Ben’s explanation of how Erlangs fit into this was well done.
Author: McGraw-Hill Illustrated Telecom Dictionary