Any system or device connected to a network is also called a node. For example, if a network connects a file server, five computers, and two printers, there are eight nodes on the network. Each device on the network has a network address, such as a MAC address, which uniquely identifies each device. This helps keep track of where data is being transferred to and from on the network.
A node can also refer to a leaf, which is a folder or file on your hard disk. In physics, a node, or nodal point, is a point of minimum displacement or where multiple waves converge, creating a net amplitude of zero.