Modern items, practises, nomenclature, etc refers to anything that would have been unknown to a native person on the period in history for chronological reasons.
SCA and Modern
In an SCA context, modern can refer to anything after 1600 A.D, i.e., out of period. It tends to be more specific and might mean present-day, or nearly so. It would be difficult to find agreement on what the earliest modern event was.
The SCA sometimes uses the term modern to describe others. This term is less loaded than the related word mundane. As such, it is probably more gracious to refer to people not in the SCA as moderns rather than mundanes.
Re-Enactment and Modern
What constitutes modern is a lot more varied in re-enactment groups due to the focus of individual groups. Anything that appears after a groups stated period is thus considered modern.
Modern is not a term used to describe non-re-enactors. In the UK the acronym MOP or member of the public is used. In Australia the very PC non-re-enactor is often used. In jest they might also be called normals.