A collision domain is anywhere that packets can collide in the network. This generally occurs in half-duplex hub environments. So, a hub does represent a single collision domain. All packets sent out are competing for the same bandwidth. In a full-duplex switched environment, packets can be sent at the same time by multiple hosts and do not collide. Strictly speaking, in a switched environment, each port represents a segment. So, a segment can be said to be a part of the network where all devices use the same physical connection. If a group of computers were connected to a hub which then connected to a switch port, this would be 1 segment. If, however, you connect another switch, this would create another segment. You will also see the term segment used to indicate a part of the network separated from the rest of the network by a Layer 3 device (router). The following link might help.

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a00800a7af3.shtml

Hope this helps. If so, please rate the post.

Brandon