Part or all of this entry has been imported from the 1913 edition of Webster's Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing.
Origin of velarium
Latin vēlāriumfromvēlumsail, sheet, curtain
American Heritage Dictionary of the English Language, 5th Edition
We learn also that it was permanently covered, and it was probably used for musical entertainments, but in the case of the larger theatre also the arrangements for the occasional extension of an awning (velarium) over the whole are distinctly found.
The medusoids have no true velum; in some cases a structure more or less resembling this organ, termed a velarium, is present, permeated by endodermal canals.