What country was Slovakia in a federation with until 1993?

Explanation

Slovakia is a republic in Central Europe that until 1993 formed part of a federation with the Czech Republic. The country is mountainous with lowlands towards the south east by the river Danube. It covers an area of 48,845 km² and has a population of 5.4 million which is primarily made up by Slovaks and Hungarians, with a significant Roma minority. The country is land-locked and shares borders with Austria, Hungary, Ukraine, Poland and the Czech Republic.