A node is a basic unit used to build data structures, such as linked lists and tree data structures. Nodes contain data and/or links to other nodes. Links between nodes are often implemented by pointers or references.
In graph theory, a graph describes a set of connections between objects. Each object is called a node or vertex. The connections themselves are called edges or arcs.
In physics, a node is a spatial locus along a standing wave where the wave has zero amplitude. This has implications in several fields. For instance, in a guitar string, the ends of the string are nodes. By changing the position of one of these nodes through fretwork, the guitarist changes the effective vibrating length of the string and thereby the note played.
In astronomy, a node is one of two points where a body's orbit crosses the ecliptic, called the ascending node (when the body is moving northward) and descending node (when the body is moving southward). The lunar nodes are subject to lunar precession, this is the gradually shift westard because of the gravitational pull of the Sun upon the Moon. (See also: eclipse)