1. Part of a fly system. This is the steel frame which holds the weights. In a single purchase counterweight fly system, the arbor is weighted to equal the weight of the batten, lift lines, and load, if any. It travels up and down opposite of the batten to counter balance the batten. The arbor is connected to a loop of rope called the purchase line, which is used to operate the fly system. At the top of the arbor a built-in clew connects all of the lift lines from the batten to the arbor. See also http://www.controlbooth.com/forums/...weight-fly-system-rigging-sub-glossaries.html, and the collaborative article: http://www.controlbooth.com/forums/collaborative-articles/5576-counterweight-rigging.html. 2. The part of a saw that holds the circular blade. 3. A trellis or arch supporting vine-type plants intended to be walked under. 4. Relating to trees, as in "arborist" or "Arbor Day--the last Friday in April".