1. A semi-automated system for generating query expansion rules comprising: memory which stores: a candidate detection module, which automatically extracts candidate synonymy pairs from expressions used by users in queries when searching an associated knowledge base, the user queries being grouped into user sessions based on session information, each user session including a first user query input by a user and a subsequent reformulated user query input by the same user, the candidate detection module generating each synonymy pair from a term of a first user query input by a user and a term of a reformulated user query input by the same user in the same user session; an interactive definition module for defining synonymy rules based on the extracted candidate synonymy pairs; and a synonymy rule export module which exports query expansion rules based on the defined synonymy rules to a storage medium for use in expansion of user queries when searching the knowledge base; and a computer processor which executes the modules.