Anything that indicates future trends.
A stock or bond that is widely believed to be an indicator of the overall market's condition.
The leading sheep of a flock, having a bell hung round its neck.
A precursor or harbinger, a warning ahead.
By extension, a non-competitor who leads out the competitors on to the circuit, or who runs/rides the course prior to competitor trials, usually testing or checking the way.
A postage stamp used in the time before a region or area issues stamps of its own.
A runner at the front or ahead.
A forebear, an ancestor, a predecessor.