kuudessa
Kuudessa is a Finnish grammatical form rather than a standalone concept. It is the inessive case (the “in” form) of the numeral kuusi, which means six. In Finnish, numerals decline like nouns, taking case endings to indicate space, time, or other relations. The form kuudessa is created from kuusi plus the inessive suffix -ssa, and it appears in phrases that express something happening within or located inside six units.
As a case form, kuudessa is used in contexts such as time expressions (and occasionally spatial ones)
Kuudessa is related to other numerals that take the same -ssa/-ssä in the inessive for non-plural counting