noun

• (religion) The doctrine that all religions are equally valid.
  "The second class of parishioners were less Philistine, certainly, but even more hopeless from the parish priest’s point of view. In their luxurious houses they lived an easy, selfish, and sensual life, beyond his reach, surrounded by a wall of indifferentism, and contemptuous of all that was not tangible and material."
• (more broadly) Relativism, agnosticism; apathy, indifference.
  "A creature, so to speak, only half awake, or awake, perhaps, only when she devoured her books and tried to puzzle out her mathematical problems; and going through life by the side of her jealous, brutal, sickly, drunken husband, in a kind of somnambulistic indifferentism, perhaps not feeling her miseries very acutely, and probably not envying other women their meaningless liberty, their inane lovers, their empty wholeness of life."
• (countable) An expression of such a doctrine or view.
  "Here and there you may still see a man—even a youth—with a single eyeglass, an elaborately bored and weary air, and a little stock of cynicisms and indifferentisms contrasting oddly with a mortal anxiety about his clothes."
• (philosophy) The doctrine of absolute identity, i.e. that to be in thought and to exist are one and the same thing.