Baby walker: A device that allows a baby to move about in a half-seated, half-upright position.
A "walker" may paradoxically delay a baby in development. The baby using a walker tends to be slow in reaching motor milestones such as sitting up, crawling, and walking and other milestones such as those involving memory, learning, and language skills.
The American Academy of Pediatrics advocates a "ban on the manufacture and sale of mobile infant walkers in the US." The AAP also stresses that parents should be made aware of the "lack of benefits" of baby walkers.
Source: Journal of Developmental and Behavioral Pediatrics 1999;20:355-361.