The shaft to which the circular saw blade is attached to and rotated by a power saw Motor.
Etymologically, arbor is borrowed, circa 1660 from the French arbre, which itself is originally Latin, tree, hence arbor.
According to OED:
a. "The main support or beam of a machine (e.g. of a crane or windmill)", or b. "The axle or spindle on which a wheel revolves, esp. in clocks and watches".1659
J. LEAK, Water-works 28
To the Arber of the said Pinion there shall be a Wheel having 32 Teeth.
1727-1751 Chambers' Cyclopedia
Crane The modern crane consists of several members... the principal whereof is a strong perpendicular beam, or arbor.
1759 Samuel Pullein, "A new improved Silk-Reel", Philosophical Transactions of the Royal Society of London, Volume 51, page 27
... "as the reel turns, it gives motion to the two wheels B and C, which are fixed upon one common arbre".
CRAIG Arbor-chuck..a chuck, consisting merely of a spindle, generally made of metal, projecting from the mandril of the lathe, used in turning and polishing rings, hollow cylinders, etc.
DENISON Clocks & Locks 4 The prolonged arbor of the centre wheel. which the Blade mounts, or, more often, on its own assembly, driven with a Belt(s) by the Motor.
In the illustration on the left -- it's from Herman Hjorth's article on "How to Operate Your Power Tools: The Circular Saw, Part 1", Home Craftsman 17 May-June 1948 , pages 24-26, 52-54 -- the arbor is on the right side.
An arbor resembles a Shaper's Spindle, the metal rod that drives the Shaper Cutter.See Trunnion and Tilting Arbor/Tilting Table. (more on this comparison later 2-10-07)
Today, in light of the fact that most large Routers include accessories like a Router Table and router Collets accommodate Bits with 1/2" Shanks, the features traditionally defined the differences between Shapers and Routers are becoming blurred. On the left, for example -- a 1/2" arbor assembly designed for a large router's collet, and currently available from several distributors like CMT and Grizzly -- smaller scale shaper cutters are mounted on routers.(What needs to be understood, though, is the exceptional speed obtained by the router's Universal Motor -- it rotates normally at 21,000 to 24,000 RPMs -- could make these "arbors" potentially dangerous; a shaper's cutter is designed for 10,000 or less RPMs, with at least a 3/4" spindle. To use this 1/2" arbor in a router safely, the rotation speed of the router should be reduced to at least 10,000 RPMs.)
In the Shaft/Arbor/Spindle mechanism on such combination tools as Shopsmith and Supershop, illustrates graphically the concept of how Arbors and Spindles perform similar functions, but with different sorts of cutting edges:
The arbor -- usually in a horizontal plane -- with the circular saw blade, the stacked dado, the molding head. The spindle, on the other hand, functions most frequently in a perpendicular position, as in the shaper, the drill press.(Yet to be accounted into this equation is the lathe and the jointer/planer, where the Shaft/Arbor/Spindle mechanism is, for the most part, on a horizontal plane. See my discussion about the Evolution of the Woodworker's Cutting Edge.