Ostiarius is the Latin form of the English ostiary. Neither word is common nowadays. It is derived from Latin ostium, 'an opening'. , such as a door or gate[way] -which is itself derived from os, 'a mouth'. It has been regarded as an equivalent of 'porter' - for example,, the servant who controls the entrance to the castle in Shakespeare's Macbeth. (It is derived from Latin porta, 'door', via French porte.
- The literal meaning is 'door-keeper'. (It gives rise, in the Late [Latin]] form ustiarius, to the modern English 'usher'.)
- In ancient Rome, large households maintained a special slave, the ostiarius, to be in charge of the entrance - and indeed the entrants. The basilicas of ancient Rome, the courts of justice, had ostiarii (the plural) who controlled access to the courts and judges.
- When the early Christian church began to use buildings as churches, and to imitate the design of the basilica, they too employed ostiaries to guard the entrance and control admission. Other duties followed. By 258CE (when St Romanus Ostiarius was martyred, the post was recognized - and by 377 ostiaries were recognized as the lowest of the Holy Orders of the Roman Catholic church, until they were abolished in 1972.