IPA: /ˌpɒlɪˈtoʊnəli/
KK: /pɑːlɪˈtoʊnəli/
In a manner that involves multiple tonalities or musical keys at the same time.
The composer wrote the piece polytonally, creating a rich and complex sound.
Polytonally → It is formed from "poly-" (from Greek "polus", meaning many) and "tonal" (from Latin "tonalis", meaning relating to tone) and the suffix "-ly" (meaning in a manner). The word "polytonally" refers to a manner of using many tones or pitches in music.
Think of 'many' ('poly-') tones ('tonal') being used in a certain way ('-ly') — that's what polytonally means.