Wiktionary
n. The practice of creating and publishing brief posts that can include short sentences(140-200 characters), images, or video links on a personal blog on a microblogging Web site. Example: a “tweet” on Twitter would be a microblog post. vb. (present participle of microblog English)
Wikipedia
Microblogging is a broadcast medium that exists in the form of blogging. A microblog differs from a traditional blog in that its content is typically smaller in both actual and aggregated file size. Microblogs "allow users to exchange small elements of content such as short sentences, individual images, or video links", which may be the major reason for their popularity. These small messages are sometimes called microposts.
As with traditional blogging, microbloggers post about topics ranging from the simple, such as "what I'm doing right now," to the thematic, such as "sports cars." Commercial microblogs also exist to promote websites, services and products, and to promote collaboration within an organization.
Some microblogging services offer features such as privacy settings, which allow users to control who can read their microblogs, or alternative ways of publishing entries besides the web-based interface. These may include text messaging, instant messaging, E-mail, digital audio or digital video.