The play by a defender of an unnecessarily high card on the first round of a suit followed by a lower card on the second round to signal to partner.
Usually the peter shows that the signaller likes the suit or that he has an even number of cards in the suit. ‘Peter’ and, especially in America, ‘Echo’ are synonymous terms for this signal. For example, if, in a suit contract your partner cashes an ace- king, you would usually play high-low with a doubleton to signify that you are happy to have a third round led to ruff.