Worship, technically speaking, as opposed to slang usage:
The peculiar collection of physical rituals and/or constraints which are employed when a soul bound in the terrestrial plain attempts to communicate with a spirit.
Terrestrial souls are not separated from the minds of spirits in the same way that they are separated from each other. Because of this difference, communication with a spirit requires more concentration. The physical feedback of sight and sound (usually present in interactions with other physical persons) are more lacking. In order to aid concentration, many worshippers impose physical disciplines upon themselves. The discipline of the physical body keeps it from intruding as much upon the spiritual communion.
Worship is an extremely powerful biofeedback mechanism, and is often used in therapy. Some type of worship is employed by almost any person who believes in spirits -- even the set of rules associated with superstition can be thought to be worshipful. Worship is not necessarily related to the idea of a God, though Gods are quite commonly beloved of worshippers.
Within any belief system that embraces the idea of a dual human being, both spiritual and physical (if not more multi-faceted), worship is regarded as developmental of the spirit.