A small, sharp nail with a flat top. Also, to fasten something with a tack.
Tacks are often used to hang things on walls or to hold fabric together.
Examples:
I stepped on a tack and it hurt!
She used a tack to hang up the picture.
The tack is on the table.
Don't sit on the chair, there's a tack on it!
There were so many tacks on the wall, it looked like a porcupine.
He carefully tacked the paper to the wall.