Slavic an Slavonic are used interchangeably in English, with the former preferred in U.S. English, and the latter in UK English. The Oxford English Dictionary gives citations of Slavonic back to the mid-17th century, whereas it seems that Slavic only appeared in the 19th century. Derivatives can be made from both forms, e.g. Slavism vs. Slavonism and Slavicize vs. Slavonize; but note Slavification vs. Slavonisation.