The top part of an iron blade, running from heel to toe visible to the golfer at address.
as viewed from address position, the top of the clubhead. Thick toplines tend to help players hit the ball higher and farther at the expense of workability.
The top-most portion of an iron's head profile that is visible at address. The topline of a club can range from wide, common in many "game improvement" clubs to promote confidence at address, to thin, popular in clubs designed for lower handicaps to provide a more traditional look at address.