1 : a drawing created by superimposing a semitransparent sheet of paper on the original image and copying on it the lines of the original image
2 : a track or mark left by something that has passed
3 : an element that occurs at very small quantities in the body but is nonetheless important for many biological processes
4 : the trail left by a person or an animal; what the hunter follows in pursuing game
5 : a just detectable amount
6 : a suggestion of some quality