OSTIA´RIUM one form of Tributum, Capitis [see VECTIGALIA]. A tax imposed in Cilicia, Syria, and perhaps some other provinces, upon doorways, whose number was probably regarded as a sign of the value of the property. (Compare the English window-tax.) Nothing is known of the amount. The word ostiarium is found in Caesar, 3, 32. Cic. Fam. 3.8, 5, speaks of exactio ostiorum.