From OpenCog

The LexicalNode is a type of Node used in constructing Connectors. It is meant to denote items that can be found in a lookup table or "lexis".

A special case of the LexicalNode is the BondNode, which is almost the same thing, except that the BondNode is more useful when the lexis is finite, small and "closed", as opposed to large and "open".

As an example, the BondNode provides the base class for TypeNode. In this example, the TypeNode holds the simple types of the system, and the set of simple types is bounded and small (e.g. there is a wiki page for each of the simple types, you're reading one now!)

See Connectors and Sections for details.