The window where a card charges no interest.
A grace period is the time between the statement date and the payment due date — usually 21 to 25 days — during which a purchase balance paid in full carries no interest.
Carry any balance and the grace period disappears on most cards: new purchases start accruing interest from the day they post, until the card is cleared in full again.
Cash advances and most balance transfers never get a grace period at all.
Clear the card inside the grace period and a $2,000 month of spending costs nothing. Miss it once at today's 20.94% APR and the same $2,000 quietly costs about $35 in the first month alone.
See the card apr page