This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

ad. Separated by a diameter, on exactly the opposite side. Frequently used in the phrase diametrically opposed meaning absolutely in opposition.

ad. In a diametrical manner; directly.

In a diametrical direction; directly; in an extreme degree.

ad. as from opposite ends of a diameter

"The gorgeousness is in the imagery not in the language; the words are weak while the sense, as in the classical Scandinavian books, is strong; and here the Arabic differs diametrically from the florid exuberance and turgid amplifications of the Persian story-teller, which sound so hollow and unreal by the side of a chaster model."

directly   

geometrically    metrically    symmetrically