Global developmental delay is an umbrella term used when children are significantly delayed in two or more areas of development. It can be diagnosed when a child is delayed in one or more milestones, categorised into motor skills, speech, cognitive skills, and social and emotional development.[1] There is usually a specific condition which causes this delay, such as Cerebral Palsy, Fragile X syndrome or other chromosomal abnormalities. However, it is sometimes difficult to identify this underlying condition.[2]

Reference
Wiki: Global developmental delay