An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) A person or thing that comes before another of the same kind; a forerunner.
2) A substance from which another is formed, especially by metabolic reaction (OxforddDctionaries.com)

M.E., from L. præcursor "forerunner," from præcursus, p.p. of præcurrere, from præ- "before," → pre-, + currere "to run," → current.

Pišgâm, from piš- "before," → pre-, + gâm "step, pace," → stage.

A component of a → pulsar pulse that appears shortly in advance of the main pulse.

→ precursor; → pulse.