Arecidae is a botanical name at the rank of subclass. Circumscription of the subclass will vary with the taxonomic system being used (there are many such systems); the only requirement being that it includes the family Arecaceae.
The Takhtajan system[1] used this name for a subclass in the class Liliopsida (=monocotyledons).
superorder Arecanae
order Arecales
order Cyclanthales
family Cyclanthaceae
order Arales
family Araceae
family Lemnaceae
order Pandanales
family Pandanaceae
order Typhales
family Sparganiaceae
family Typhaceae
The Cronquist system (1981) used this name for a subclass in the class Liliopsida (=monocotyledons), with the circumscription:
order Arecales
order Cyclanthales
family Cyclanthaceae
order Pandanales
family Pandanaceae
order Arales
family Acoraceae
family Araceae
family Lemnaceae
The APG II system does not use formal botanical names above the rank of order; it assigns the plants included in this subclass by Cronquist in various taxa throughout the clade monocots. In other words, by today's standards Cronquist's subclass was not a "good" group.