A blade may refer to any of the following:
1. When referring to a network router, switch, server, or other network device, a blade (also known as an expansion module) is a hardware component that can be installed into a chassis. It allows for additional functionality, much like an expansion card in a computer, but often at a much bigger scale. For example, a router or switch with blade support allows complete customization, if more fiber lines are needed, additional fiber blades can be added.
2. When talking about servers, a blade, blade server, or a high density server is one rack mount unit that is capable of holding dozens of different servers. Blade servers can save space and make maintaining a network of hundreds of servers a lot easier. The picture is an example of the Dell PowerEdge 1855 and what a blade server may look like outside of a rack.