[1] Weather helm is the result of a leeward and aft shift of a vessel's vector center of effort (the direction to which the force generated by the sails is pushing).
This shift is caused by excess pressure on the mainsail, which overpowers the windward lateral resistance generated by the jib (or other head sail) and keel or centerboard.
[2] Weather helm does not only result from an overpowered main; when a vessel is heeling to leeward, the aft component of keel drag is moved to windward.
In keelboats, despite the fact that weather helm is not as readily felt, it can be just as detrimental, as the lateral drag against the blade still exists, along with the need to pull the rudder to an undesirable position (see Mitigation).
Yachts making ocean trade wind crossings have rigged 'twins' - double headsails poled out to opposite sides from the same forestay for extended downwind passages without a mainsail.
In the book "Sailing Illustrated", Patrick M. Royce defines weather helm as simply a "heeling sailboat wanting to come head to wind.