Phase cloak

A phase cloak -- also known as an interphase generator -- is a device shifts matter "out of phase" with its environment, making it largely non-interactive with normal matter and consequently invisible and intangible.

Nature

The phase cloak first appeared in TNG "The Next Phase", when a malfunctioning Romulan prototype caused two members of the Enterprise's crew -- Geordi Laforge and Ro Laren -- to disappear. The device made them invisible, inaudible, intangible, and essentially undetectable, but they did cause exotic particles to appear whenever they moved through solid objects, allowing them to eventually demonstrate that they were alive and be restored to their normal state by another type of particle.

A fully-functional Federation phase cloak appeared in TNG "The Pegasus". While the device had caused a catastrophic explosion on the ship that was testing it, it was eventually installed on the Enterprise with a few modifications and worked flawlessly. The ship became invisible, passed freely through a few kilometers of solid rock to escape an asteroid, and completely foiled the sensors of a Romulan Warbird.

Unrealism

In both instances, the effects of the phase cloak were totally implausible. Geordi and Laren were invisible, but they could still see (interaction with light is required for vision). Geordi and Laren were inaudible, but they could still hear normally and even speak to each other (interaction with air is required to make or hear sound -- not to mention breathe, which they were also doing normally). Geordi and Laren could walk through walls, but they could also walk on the deck and ride in shuttlecraft (interaction with solid matter is necessary to walk or even stand).

In terms of realism, this episode is a sick joke. I can conclusively demonstrate from their observed abilities (seeing, hearing, walking) that they are both physically and chemically interactive with normal light and matter, on both microscopic and macroscopic scales. In other words, they're just like you and me, but with technobabble. Mass hypnosis almost seems to be a better explanation for their apparent invisibility than "phasing". -- Mike Wong commenting on "The Next Phase"

Similarly, the phase-cloaked Enterprise was invisible and undetectable to Romulan sensors, but its crew could still see out the windows, and they could use their own sensors normally.