Filled, outline and sized squares
As with circles, Unicode calls filled shapes "black" and outlines "white": ■ is the black square (U+25A0) and □ the white square. They come in several sizes, from the very small ⬝ through ▪︎, ◾︎ and ◼︎ up to ■ and the large ⬛︎. ▢ has rounded corners, ▣ has a small square inside, and ⬚ is drawn with a dotted outline.
A few squares carry meaning. ∎ (end of proof, also called the tombstone or Halmos symbol) marks the end of a mathematical proof, like QED. The squared operators ⊞ ⊟ ⊠ ⊡ come from math too. Note that ⬛︎ ⬜︎ ◼︎ ◻︎ ◾︎ ◽︎ can also appear as emoji, so they may look different on phones.
Colored squares, Wordle and pixel art
The colored emoji squares 🟥 🟧 🟨 🟩 🟦 🟪 🟫 are perfect for simple pixel art, color swatches and status grids. Wordle made them famous: its share grid uses 🟩 for a letter in the right spot, 🟨 for a letter in the wrong spot and ⬛︎ or ⬜︎ for a letter that is not in the word.
Text squares work as bullet points and checkboxes too: ▪︎ makes a neat small bullet, and □ or ▢ an empty box. For boxes with shadows and ticks, see the box symbol page, and for loading bars made from ■ and □, see the loading bar symbols.
How to type square symbols
On Windows, hold Alt and type 254 on the number pad for ■, or 22 for the bar ▬. On a Mac, open the Character Viewer with Control + Command + Space and search "square". In Microsoft Word, type 25A0 (■) or 25A1 (□) and press Alt + X. In HTML, use ■ for ■, and in LaTeX use \blacksquare and \square from amssymb.
Looking for something else called "square"? The superscript two in m² is on the squared symbol page, and √ is on the square root symbol page.