A monocular is an NV product that has one eyepiece and one objective. A NV monocular has many advantages and is one of the most versatile night vision devices. They are typically thought of as hand held units; however many can be head mounted as well as weapon mounted. One of the major benefits of a monocular over a binocular or biocular is the fact that you always have one eye that is adjusted to the ambient lighting conditions. This is particularly useful for users that might constantly be moving from dark conditions to lighter conditions where night vision might not be required.
A NV binocular is much the same as a daylight binocular in that it offers two eyepieces and two front lenses with the addition, of course, of an NV intensifier tube. An NV binocular still only uses one intensifier tube system, so it won't offer greater performance or range than a monocular, but it will provide more comfortable viewing since you are you using both eyes. However, an NV binocular will also be heavier and more expensive than an NV monocular.
An NV goggle is any unit that comes with a headset. It comes in three versions:
Monocular goggle - As the name suggests, this goggle uses a monocular mounted to a headset. This is the lightest and least expensive type of night vision goggle (night vision system), but since you are using only one eye and one optical system, it will not be as comfortable to use as a system with two eyepieces, nor will it provide any depth perception.
Bi-ocular goggle - The bi-ocular goggle feature two eyepieces, but only one front lens. This improves viewing comfort over a monocular goggle, but because there is only one front lens, there is no improvement in depth perception over a monocular.
Binocular goggle - The binocular goggle uses two eyepieces and two front lenses. This not only provides better viewing comfort than a monocular, it also provides depth perception since it is using two different optical systems. This is an important feature if you are using a goggle while in motion, say walking or even driving off road.