Real Dictionary

yo-yo
countable noun: A yo-yo is a toy made of a round piece of wood or plastic attached to a piece of string. You play with the yo-yo by letting it rise and fall on the string.

yo-yo in American English
a spool-like toy attached to one end of a string upon which it may be made to spin up and down
to move up and down; fluctuate; vary
up-and-down; fluctuating; variable
adjective: moving up and down or back and forth; fluctuating; vacillating
intransitive verb: to move up and down or back and forth; fluctuate or vacillate
noun: a spoollike toy consisting of two thick wooden, plastic, or metal disks connected by a dowel pin in the center to which a string is attached, one end being looped around the player's finger so that the toy can be spun out and reeled in by wrist motion
transitive verb: to cause to yo-yo

yo-yo in British English
adjective: changing repeatedly; fluctuating
noun: a toy consisting of a spool attached to a string, the end of which is held while it is repeatedly spun out and reeled in
verb: to change repeatedly from one position to another; fluctuate