trace
n. C / U C1 Advanced Oxford US //ˈtɹeɪs// UK //tɹˈeɪs// trace
n. a very small amount of something that stays behind. It can also mean a mark or sign that shows someone or something was there.
n. an extremely small amount of a substance, or a visible mark or sign of a past presence or event.
The police found a trace of blood on the floor.
The laboratory test was sensitive enough to detect a tiny trace of the chemical in the water supply.
Archaeologists discovered a faint trace of an ancient wall, suggesting that the settlement was much larger than previously estimated by the research team.
Usage
Often used in the plural ('traces') when referring to physical evidence, but singular when referring to a chemical amount.