According to the 6th edition of the APA manual, an online encyclopedia falls under the general heading of "electronic sources." As such, the online encyclopedia reference should include the associated URL. Also, since online encyclopedia entries don't usually include the author's name, the listing should begin with the entry name. The in-text citation should reference this entry name and the entry's date of publication, or "n.d." if no date is present.
A parenthetical citation should use the format ("Entry Name," date) when a publication date is available, or ("Entry Name," n.d.) if no date is present. A signal phrase should follow the same guidelines.
A parenthetical citation example: The bush elephant is also known as the African savanna ("Elephant," n.d.).
A signal phrase example: According to the Encyclopædia Britannica's "Elephant" entry (n.d.), the bush elephant is also known as the African savanna.
In the reference list, begin the encyclopedia listing with the entry name. If a publication date is not available, use "n.d.".
Elephant. (n.d.). In Encyclopædia Britannica online (in italics). Retrieved from http://www.britannica.com/EBchecked/topic/184366/elephant