One half of the Moon is always illuminated, except during a lunar eclipse. But we only see part of it.
You may be referring to the new moon, when the half that is lit up is facing away from the Earth, and the half that faces Earth is dark.
The waxing phase of the moon is the period of time between new moon and full moon, where the size of the illuminated portion of the moon is increasing. The waning phase of the moon is the period of time between full moon and new moon, where the size of the illuminated portion of the moon is decreasing.
The crescent phase that precedes the new moon phase is called the waxing crescent, where the illuminated portion of the moon is increasing. The crescent phase that follows the new moon phase is called the waning crescent, where the illuminated portion of the moon is decreasing.
The waxing phase of the moon is the period of time between new moon and full moon, where the size of the illuminated portion of the moon is increasing. The waning phase of the moon is the period of time between full moon and new moon, where the size of the illuminated portion of the moon is decreasing.
The phase of the moon when it is fully illuminated and appears as a whole circle is called a "full moon." This occurs when the Moon is on the opposite side of the Earth from the Sun.
After a full moon, the illuminated portion of the moon gradually decreases as it moves toward the last quarter phase. This period is known as the waning phase, during which less of the moon's surface is lit by the sun. The illuminated half decreases until it reaches the new moon phase, where the moon is not visible from Earth.
When the side of the Moon facing Earth is fully illuminated.
One week after the full moon, the moon is in its third quarter phase. During this phase, the left half of the moon appears illuminated when viewed from the Northern Hemisphere, while the right half appears illuminated from the Southern Hemisphere. This is because the sunlight is shining on the opposite side of the moon compared to the full moon phase.
Full Moon. A New Moon is when you can't see a moon at all.
The Waning Phase in the lunar cycle occurs as the moon shrinks from the Full Moon to the New (Dark) Moon. Note: You can tell when you are in the Waning Phase when the Left Side of the Moon is illuminated. Conversely, if the Right Side is illuminated, the Moon is Waxing.
When the moon appears as a half-illuminated disk, and the illuminated portion is growing from one night to the next,the phase is known as "first quarter".
The moon's surface is always half illuminated. We on Earth see 0 to 100% illumination of the side that faces use, depending on what phase the moon is at. Quarter phases, first and last, are half illuminated when seen from Earth.
During the new moon phase, the side of the moon facing Earth is not illuminated by the sun, making it impossible to see.