Black Lozenge

A filled diamond-shaped marker that means nothing beyond its shape. A lozenge is a rhombus standing on one point, usually drawn taller than wide, the outline of a throat lozenge or a playing-card diamond. ⧫ is that shape filled in black. Its hollow partner is ◊, an older typographic character; TeX users know the pair as \lozenge and \blacklozenge.

In maths it is a general-purpose symbol: an author may define it as an operator, use it to tag a special case, or occasionally close a proof with it in place of the usual ∎ or □. The hollow ◊ has a famous job in modal logic, where it means "possibly"; the filled ⧫ has no standard role of that kind. Outside maths it turns up as a fancy bullet, a divider between words in a username or bio, or a small black diamond in a title.

What does ⧫ mean when someone sends it?

Nothing in particular; read it as a decorative diamond. People copy it from a symbol list to dress up a name, separate lines, or mark items in a list, much like ◆ or ♦. It is not a secret code and carries no fixed mood, though a row of black diamonds can look deliberate and a little sharp-edged. If it arrives alone with no words, the sender most likely tapped a shape they liked; ask them rather than hunt for a meaning that is not there.