Is labarum a word in the Scrabble Dictionary

Yes, labarum is playable

...and is worth 15 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Labarum" Definition

noun

An ecclesiastical banner, especially one carried in processions.
The banner adopted by Constantine I after his conversion to Christianity.
An imperial standard of the later Roman emperors resembling the vexillum.
The standard bearing the Chi-Rho adopted by Constantine after he converted to Christianity.