Answer:
The word is "invisibilis." What form this takes in any given situation depends on how it's used in the sentence. The possibilities are:
invisibilis: masculine/feminine nominative singular; genitive singular all genders
invisibile: neuter nominative/accusative singular
invisibilem: masculine/feminine accusative singular
invisibili: dative/ablative singular all genders
invisibiles: masculine/feminine nominative/accusative plural
invisibilia: neuter nominative/accusative plural
invisibilium: genitive plural all genders
invisibilibus: dative/ablative all genders