An Altermere is a species in the W.I.T.C.H. universe
Comics
In the comic series, an altermere is a symbol of power. It is created when parts of a power have been combined, but not all of it. The altermere will seek out the last piece of its power until the power is fully united.
One was created when Luba forced the Aurameres together to try and save them from destruction. The Altermere took Will's, Irma's, Taranee's, and Hay Lin's powers, and found its way to Cornelia, uniting the power within her and freeing Nerissa
TV-Series
In the Animated series, an altermere is an Astral Drop that had been given life via Quintessence (element)
One episode was focused directly on the concept of Astral Drops and altermeres, mainly Will Vandom's. In the episode Will creates an Astral Drop to do her chores for her, but the evil sorceress Nerissa upgrades it into an Altermere, which gives it a soul of its own, as well as all of Will's feelings, emotions and powers. The Altermere lives Will's life for a short time and grows attached to it. Unfortunately, Will still thinks its a dull Astral Drop, and hunts it down to reabsorb it into the Heart, thinking it won't care. She and the other guardians corner her at the school football field. Will insults her by saying she has no emotions and doesn't feel anything, which makes her cry and attack Will out of fear of being destroyed. After the Altermere is subdued, Will finally realizes that she is alive, and releases her. The Altermere forgives Will, and they share a touching moment, but this ends when Nerissa fires a lightning blast at the two of them. The Altermere takes the blast to defend Will, but is fatally injured and begins to disappear. Will is forced to absorb her Altermere into herself so that her memories, emotions, and feelings can live on in her. This is the only real death scene in the entire series and consequently one of the saddest. It also goes to show Will is a selfless person; the altermere was made from Will and is a part of her - instinctively, the alteremere sacrificed herself to save Will, despite the two having reconciled only moments before.