The side of the coin. Currently US dimes and quarters have a "reeded" edge, which is an edge with small lines on it. Some coins will have lettering, ornamental designs, or plain edges.
Letters or designs made on the side edge of a coin. Most modern day coins have plain or reeded edges. Example of edge lettering is the old Capped Bust Half dollar coins, the $10 Indian Head and modern-day Presidential Dollars.