Tagdef.com is the worlds largest hashtag dictionary. Use this api to access these definitions. The content is user-generated, and the directory currently contains over 60.000 definitions. The hashtags can have many definitions each, ordered by user-votes.
The hashtags are often related to Twitter , but is also commonly used on Pinterest and Google+ .
This API is free to use, but you must provide a clickable link back to the tagdef.com page for the given tag when using information from the API. This link can be found in the uri part of the reply.