a device that bridges network segments, operating at the OSI link layer level.
Switches (in the context of this site) are devices with multiple network ports, which allow interaction between all devices connected to these ports.
Basic switches operate at the data link layer level (in TCP/IP networks) and forward packets based on the link layer address in the packet headers. They are different to hubs in the way that hubs generally broadcast incoming packets to all other ports, whereas switches keep tables of link layer addresses they have seen at specific ports and only forward packets to the relevant ports. Basic switches are also known as layer 2 devices.
Higher grade switches can include functionality at higher layers of the OSI stack. This can range from IGMP snooping (for multicast support) and VLANs through to NAT, VPN, firewalls or even application layer functionality (web caching or proxying). Such switches are known as multilayer devices.