From Pluralpedia, the collaborative plurality dictionary
imposition (n.)
Other formsImpose (v.), Imposed (adj.)

Imposition is, generally, the act of intentional consciously-directed hallucination. This general term has a more specific use in plural communities: the intentional hallucination of headmates. Some examples: "seeing" a headmate in the room with you, hearing their voice not just internally, but as if it was coming from another physical location, or feeling a touch.

Imposition is not drug-based hallucinations, dreams-based hallucinations, or hallucinations due to neurochemical imbalances. Rather, it refers to directed and intentional overriding of the brain to have specific non-random effects.

Any given imposition can vary in multiple dimensions. It could cover various senses, from tactile to visual to auditory and beyond. Imposition can vary in quality, from hazy or distorted, to crisp and precise. Imposition can feel more or less 'real'. One way this 'realness' might show up is by whether the given hallucination blocks out the sense, eg whether you can 'see through' an imposed headmate or not. Finally, Imposition can be controlled by the front, the headmate in question, a separate co-conscious headmate, or some combination.

Imposition is very much a skill, and often a highly desired one in the plural community. Interacting with headmates in the real world can feel quite different to interacting in headspace. Imposition against different senses often requires separate training, however, and as such training imposition can be a lifelong process of finetuning and upkeeping.

It can also be performed entirely separate from plural context, even by singlets. Intentional hallucinations of touch from text are a rather common example for internet communication.