Loft
The angle of the clubface relative to vertical - the primary lever that controls how high and how far the ball flies.
Loft is the angle between the clubface and a vertical line, measured in degrees. More loft sends the ball higher and shorter; less loft sends it lower and farther. It is the most important number on any club because it governs launch and distance more than any other single spec.
Every club in the bag is defined by its loft. A driver typically runs 8–12 degrees, irons climb from roughly 20 degrees in a long iron to 45 in a pitching wedge, and wedges go from there up to 60 degrees and beyond. The gaps between those lofts are what create consistent distance gaps between clubs.
Loft has crept stronger over the past two decades. A modern 7-iron may carry the loft of a 5-iron from twenty years ago, which is why "my 7-iron goes 165" means less than it used to. Stronger lofts add distance but need more clubhead speed to launch properly - which is part of why fitting matters.
Loft can be adjusted. Most modern drivers and fairway woods have adjustable hosels that move loft up or down a degree or two, and irons can be bent stronger or weaker by a club technician. Small changes have real effects on launch and gapping.