Memoriam is a Background in Wraith: The Oblivion. Wraiths with this background can tap into the emotions and memories that the living hold for the Wraith, whether they be positive or negative. The more people who remember the wraith, the more Memoriam that the wraith has access to.
A Wraith with this background can tap into their Memoriam as a reservoir of pathos. However, memories eventually fade. A wraith who botches a Memoriam roll loses a point of this background permanently, representing the frailty of human memory.
References
WTO: Wraith: The Oblivion Second Edition 
, p. 131