Semantic similarity and definition of words are very crucial part of the data; therefore various online dictionary and thesaurus had been studied in the literature and it was found that wordnet is the most famous and better one. WordNet design is inspired by current psycholinguistic theories of human lexical memory. It is an ontology-based system which organizes English nouns, verbs, and adjectives into synonym sets, each representing one underlying lexical concept. Although wordnet is a more efficient dictionary and thesaurus then also there are lots of problems in it which make it not so good. Therefore in this paper we will discuss all the challenges of existing online dictionaries and thesauruses and then we will conclude that why WordNet is good but not the best system. This state of the art paper clarifies the difference between all existing online dictionaries and thesauruses by using an effective parameter.