⬜ · Symbols › Shapes · Emoji 0.6 (2010)
⬜ is a solid white square, one of the original emojis from 2010.
In light-mode Wordle results it marks a letter that isn't in the word. In emoji art it's the empty pixel, the background, clouds and snow, and in progress bars it's the part still to go.
Paint with ⬜ in the emoji art editor, or turn any photo into emojis with the image to emoji converter.
U+2B1C⬜\2B1C\u{2B1C}\U00002B1C%E2%AC%9C← ⬛ Black Large Square · ◼️ Black Medium Square → · Symbol emoji library