The term 'firmament' is used to refer to the SKY or HEAVENS, and is found in many different crossword solutions. The seven-letter solution is 'HEAVENS, ' which is a synonym for the SKY or the expanse of space above the earth. The five-letter solution is 'VAULT, ' which can be used to describe the SKY as a protective covering or canopy. The eight-letter solution is 'EMPYREAN, ' which is an adjective meaning 'of or relating to the highest HEAVEN.' The six-letter solution is 'WELKIN, ' which is another word for the SKY or HEAVENS. The six-letter solution is 'HEAVEN, ' which is a synonym for the SKY or the expanse of space above the earth. The three-letter solution is 'SKY, ' which is another word for the firmament. The thirteen-letter solution is 'vaultofheaven, ' which is a combination of the words 'VAULT' and 'HEAVEN' to refer to the SKY. The fifteen-letter solution is 'celestialsphere, ' which is a combination of the words 'CELESTIAL' and 'SPHERE' to refer to the SKY or HEAVENS.