An acetylated (has a CH3CO- group attached) form of serine, one of the twenty naturally-occuring amino acids, which is thought to initiate translation of mRNA into a chain of amino acids in mammals the way N-formylmethionine does for bacteria.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.