The Aeolian mode was named in the 16th century by Heinrich Glarean. In modern terms, it is known as the natural minor scale and can be described as the scale from A to A using only the white notes of the piano. In order to construct the scale starting on other notes, follow the pattern ascending from tonic: whole step – half step – whole step – whole step – half step – whole step – whole step.
A handout showing the mode starting from all twelve notes is available here.
While the Hypodorian mode also corresponds to the same A to A note range, it is considered a plagal mode counterpart to the authentic first mode, the Dorian mode.