Although some commentators warn against the use of memoranda as a singular and condemn the plural memorandas, our evidence indicates that these forms are rarely encountered in print. We have a little evidence of the confusion of forms, including use of memorandum as a plural, in speech (as at congressional hearings). As plurals memoranda and memorandums are about equally frequent.
Examples of MEMORANDUM
<I'm waiting for the memorandum that will explain the new vacation policy.>
<the studio executives depend on endless memoranda to keep track of what's going on at a movie shot on location>
Origin of MEMORANDUM
Middle English, to be remembered, from Latin, neuter of memorandus, gerundive of memorare