The Collaborative International Dictionary of English v.0.48:

Caliph \Ca"liph\ (k[=a]"l[i^]f), n. [OE. caliphe, califfe, F.
   calife (cf. Sp. califa), fr. Ar. khal[imac]fan successor, fr.
   khalafa to succed (i. e. a successor of Mohammed).]
   Successor or vicar; the civil and religious leader of a
   Muslim state; -- a title of the successors of Mohammed both
   as temporal and spiritual rulers, used formerly by the
   sultans of Turkey. [Written also calif, kaliph, kalif,
   khalif.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

caliph
    n 1: the civil and religious leader of a Muslim state considered
         to be a representative of Allah on earth; "many radical
         Muslims believe a Khalifah will unite all Islamic lands and
         people and subjugate the rest of the world" [syn: caliph,
         calif, kaliph, kalif, khalif, khalifah]