Prefer the English plural, indexes, for general uses (tab or book indexes, indexes of names or subjects) and the Latin plural, indices, for mathematical and scientific uses. Nonetheless, both plurals appear in all settings, sometimes with closely-matching frequency, leaving consistent usage advice difficult to pinpoint. Eric Partridge says no more than what I have stated; Bryan A. Garner calls indices pretentious but elaborates further on specific uses (and its spelling—not to be misspelled as indeces). Records of written use also vary greatly, from indexes being far preferred in phrases like book indexes to indices reigning supreme in phrases like financial indices. I suggest two habits: either follow my above advice—save indices for technical applications and use indexes for general ones—or stick to only the English plural.