A proper definition is of vital importance in the study of languages, which leads to the solution of such important issues as theory, target, scope, way to pursue, methodology, emphasis, result, and even adoption of language policy. However, for the past two centuries there has been a surprisingly great diversity among linguists as to what language is. This paper collects more than 60 definitions since the early 19th century, analyzes four different attitudes of the linguists towards the divergence, and finally proposes a new and tentative definition for further discussion.