marker (markers plural )
1 n-count A marker is an object which is used to show the position of something, or is used to help someone remember something.
He put a marker in his book and followed her out.
2 n-count If you refer to something as a marker for a particular quality or feature, you mean that it demonstrates the existence or presence of that quality or feature.
Vitamin C is a good marker for the presence of other vitamins and nutrients in frozen food.
3 n-count A marker or a marker pen is a pen with a thick tip made of felt, which is used for drawing and for colouring things.
Draw your child's outline with a heavy black marker.