A monde is a ball-like object located near the top of a crown. It is the point at which a crown's half arches meet. It is usually topped off either with a national or religious symbol, for example a cross in Christian countries.
When the crown is worn as a circlet, the half-arches, monde and cross detach as a unit.
The monde can clearly be seen as the jeweled ball, just below the cross, in the image of St Edward's Crown opposite.