"Aathaar" is the plural of Athar; A narration made by either a Sahaabiy or a Taabi'ee (those who came after the time of the Prophet 'alayhis salaam, there's other definitions for who a Taabi'ee is but Ive just summed it down).

Edit: Just incase you get confused. An athar is not transmitted from the Prophet salallaahu 'alayhi wasallaam.

Wallaahu a'lamu bis-sawaab.