What is a percentile?

A percentile is a value on a scale of one hundred that indicates the percent of a distribution that is equal to or below it. For example, a wieght for age of the 90th percentile is equal to or greater than 90 percent of the infants for the same age according to growth charts and the body mass index.

In general,

• a percentile greater than 75 is considered above normal
• a percentile between 25 and 75 is considered normal
• a percentile less than 25 is considered below normal

From Wikipedia:  Weight and height percentiles are determined by growth charts and body mass index charts to compare a child's measurements with those of other children in the same age group. By doing this, doctors can track a child's growth over time and monitor how a child is growing in relation to other children. There are different charts for boys and girls because their growth rates and patterns differ. For both boys and girls there are two sets of charts: one for infants ages 0 to 36 months and another for children ages 2 to 20 years old.