Typical vs atypical development of a child
Since a typically developing child is considered typical by hitting the milestones of development at the same time as other child his/her own age and gender; atypical development would be the inability to hit the milestone for a particular event such as walking. A female child who has not said her first word by the age of 2 is considered atypical within the developmental milestone of talking since females typically speak their first word by 12 months of age.
A child who shows signs of being atypically developed in multiple areas should be given a consultation by an expert to check for a disability. For example a child who has delayed speech, delayed large motor skill development, and equilibrium problems shows signs of a sensory disorder but until said child is seen by a professional in the field then a diagnosis can't occur and the child will not getting help.