1) The enzymatic addition of a sequence of adenylyl residues at the 3' end of an RNA molecule.

2) The addition of a tail of polyadenylic acid (poly A) to the 3' end of mRNA. Polyadenylation involves recognizing the processing site signal, (AAUAAA), and cleaving of the mRNA to create a 3' OH terminal end to which poly A polymerase adds 60-200 adenylate residues. The 3' end processing of some messenger RNAs, such as histone mRNA, is carried out by a different process that does not include the addition of poly A as described here.