Prefer the English plural, indexes, for general uses (tab or book indexes, indexes of names or subjects) and the Latin plural, indices, for 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 specific spelling—not to be misspelled as indeces). Records of written usage also vary greatly, from indexes being far preferred in phrases like book indexes to indices reigning supreme in terms like financial indices. I’ll offer two options: the first, and better, is to follow my above advice—save indices for technical applications and indexes for general ones; the second, and perhaps more radical, is to pick one of the two plurals and stick with it (I recommend the English plural).