interjection/ˈxaxaxa/
hahaha
“1877, José María de Pereda, Tipos trashumantes http://books.google.com/books?vid=0ufA4x4TyEYKvtjr0n&id=ZHHE6kiSqdYC&pg=PA89&lpg=PA89&dq=jajaja —Jajajá.... ¡qué buen humor tiene este señor de Zorrilla!...” — (please add an English translation of this quotation)
Onomatopoeic.