Firmament/ˈfɜːrməmənt/ Definition 1 of 3Pro Tip 1/2Visual Imagery Use 'firmament' to paint a vivid picture of the sky in literature or poetry. Poets often describe the firmament as adorned with a tapestry of stars. Pro Tip 2/2Not Scientific The word 'firmament' isn't used in modern science, it's more poetic and literary. Astronomers study the cosmos, not the firmament.