Deadheading can refer to the following: ...
Removing flowers as they fade to prevent the plant from wasting valuable energy on forming seeds and to keep plants looking tidy. Deadheading extends the life of annual_plant flowers and can prolong the flowering of perennials, especially roses.
Pinch here, snip there, removing spent flowers that have already bloomed. This is done for the benefit of the plant to prevent disease, prevent seed development and will encourage more vigorous blooming and a bushier plant.
The removal of spent flowers or flower heads.
Removing dead or faded flowers from a plant to improve its appearance prevent seed production and encourage further flowering
The process of pinching or snipping off used or spent blooms to keep the plants well groomed and to prevent them from setting seed. This will promote continued bloom.
As in removing dead flower heads from a plant to encourage reblooming, not the act of following the famous band around as a tie-dyed-clad groupie. Deadheading should be done regularly and continuously to promote a full floral display all season long.
Removing the dead blossoms. If a plant is termed “self cleaning,” the blossoms fall off on their own. Deadheading usually extends the blooming season. 
Removing faded blossom from a plant to prevent seeds from forming and to prolong the flowering season.
sounds like the sport of rock 'n' roll fans, but when gardeners do it, they cut off a plant's spent flowers.