Origin and history of miter

mid-14c., "bishop's tall hat," from Old French mitre and directly from Latin mitra "headband, turban," from Greek mitra "headband, turban," earlier a belt or cloth worn under armor about the waist, perhaps from PIE root *mei- "to bind, attach" (source also of Sanskrit mitra- "friend, friendship," Old Persian Mithra-, god name; Russian mir "world, peace"). The Greek word might be borrowed from Indo-Iranian.

In pre-Christian Latin, in reference to a type of head-dress anciently worn by inhabitants of Lydia, Phrygia, and other parts of Asia Minor, "the wearing of which by men was regarded in Rome as a mark of effeminacy" [OED]. But the word was used in Vulgate to translate Hebrew micnepheth, the sacerdotal head-dress of the ancient Jewish high priests.

word-ending that sometimes distinguish British from American English. In the U.S., the change from -re to -er (to match pronunciation) in words such as fibre, centre, theatre began in late 18c. and became standard there over the next 25 years at the urging of Noah Webster (the 1804 edition of his speller, and especially his 1806 dictionary). The -re spelling, like -our, however, had the authority of Johnson's dictionary behind it and was unmoved in Britain, where it came to be a point of national pride, contra the Yankees.

Despite Webster's efforts, -re was retained in words with -c- or -g- (such as ogre, acre, the latter of which Webster insisted to the end of his days ought to be aker, and it was so printed in editions of the dictionary during his lifetime). The -re spelling generally is more justified by conservative etymology, based on French antecedents. It is met today in the U.S. only in Theatre as an element in the proper names of entertainment showplaces, where it is perhaps felt to inspire a perception of bon ton.