A simian crease is a line that runs across the palm of the hand. This crease has a negative connotation, implying a likeness to a monkey or ape, so simian creases are more commonly referred to as singular transverse palmar creases.
Most people have three transverse palmar creases, or three lines running across their palms. Occasionally, the three lines run together and form a single crease across the palm. Roughly 10 percent of the general population have singular transverse palmar creases. The crease is much more common among people with Down syndrome; nearly half of all people with Down syndrome have a simian crease.