To become lifelong learners is the ultimate goal of many if not most learners irrespective of their target of learning, and being able to self-learn is one major prerequisite of this goal. As far as the learning of a second language is concerned, dictionaries are among the most readily available and easily accessible self-learning tools available on the market, and are regarded as the most systematic and comprehensive lexical descriptions that users have access to (Jackson & Amvela, 2000). An increasing number of dictionaries are now available for learners at different levels and for different learning purposes, including general dictionaries, learners' dictionaries, translation dictionaries, pictorial dictionaries, collocation dictionaries, specialized dictionaries and encyclopaedic dictionaries (Jackson & Amvela, 2000). Of these, learners' dictionaries are among the most popular for second or foreign language learners, providing a range of information including word meanings, pronunciations, collocations, correct usage and syntactic behaviour.