precursor

Etymology 1

Borrowed from Latin praecursor (“forerunner”). See precurse, and refer to -or.

noun

1. That which precurses: a forerunner, predecessor, or indicator of approaching events.
   Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis:[…]. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. The critical component of the photosynthetic system is the “water-oxidizing complex”, made up of manganese atoms and a calcium atom. 2013 September-October, Katie L. Burke, “In the News”, in American Scientist
2. (chemistry) One of the compounds that participates in the chemical reaction that produces another compound.

Etymology 2

From pre- + cursor.

adj

1. (telecommunications, of intersymbol interference) Caused by the following symbol.