Edudemic, which is a great resource for educators, has posted a dictionary of educational Twitter hashtags. Hashtags are ways of organizing and tagging Twitter posts and there are a lot of them for education.
Twitter is a great tool for educators and is an essential part of any educator's Personal Learning Network.
This article lists and describes the different educational hashtags. If you are an educator and use Twitter (and we all should), this is a great place to go to find the hashtags for your area of interest.