A dirigible is a steerable (directable) airship as opposed to a blimp or "balloon"
Interesting. We USns use "blimp" to mean a [steerable] dirigible, rather than a [non-steerable] balloon.
Under bartleby, http://www.bartleby.com/61/33/B0323300.html, as I read it, a "blimp" can be either steerable or non-steerable.