What is the definition or description of: microcephaly?

Smaller than average: Microcephaly is a neurodevelopmental disorder in which the circumference of the head is more than two standard deviations smaller than average for the person's age and sex. Microcephaly may be congenital or it may develop in the first few years of life. http://en.wikipedia.org/wiki/microcephaly.

Small head: Head less than the 5th percent, may be serious or just smaller than average head.

Microcephaly: Is a neurodevelopmental disorder with a head circumference more than 2-3 standard deviations below the mean for age & gender. It may be congenital or develops in the first few years of life. It stems from wide variety of conditions that cause abnormal brain growth, or from syndromes associated with chromosomal abnormalities. Life expectancy is reduced, depending on presence of other abnormalities.