A word for a “mind-picture”, a combination of indo “mind” and emma “picture” (PE17/174, 176, 179). Tolkien described them thus:

They [the Elves] held that a superior mind by nature, or one exerting itself to its full in some extremity of need, could communicate a desired vision direct to another mind. The receiving mind would translate this impulse into the terms familiar to it from its use of the physical organs of sight (and hearing) and project it, seeing it as something external ... indemmar were by Men mostly received in sleep (dream). If received when bodily awake they were usually vague and phantom-like (and often caused fear); but if they were clear and vivid, as the indemmar induced by Elves might be, they were apt to mislead Men into taking them as real things beheld by normal sight (PE17/179).

Thus this word was used for telepathically induced visions, used among the Elves for communication, but Men could sometimes be fooled (intentionally or not) into believing these visions were real if they were awake when they received them, otherwise the visions were perceived as dreams. These “sorcerous” visions were likely one of the reasons for the mistrust between Elves and Men towards the end of the Third Age.