Social indexing is an approach to manual indexing of web content. The purpose of this master thesis has been to investigate terminological and conceptual consistency in social indexing of web content. Conceptual consistency was investigated/measured focusing on synonyms and (the use of) singular and plural forms of words. In order to measure consistency in social indexing data were obtained from the Delicious website. The data set consists of five indexed objects, 3001 tags and 239 users. Synonym tags and singular and plural forms of words were identified and grouped. The indexing consistency was then calculated on five user groups, each containing twenty users. The users’ sets of tags were represented in vector spaces and each user’s vector was measured against the group’s average vector using the cosine measure. The indexing consistency was found to be low. An average terminological consistency of 15.4 % and an average conceptual consistency of 18.5 % were achieved. Synonym tags and singular and plural forms of words do affect terminological consistency but not to any great extent. The low (terminological) consistency may therefore not solely be attributed to the presence of synonyms and singular and plural forms of words among the users’ tags. / Program: Bibliotekarie