Natural / available light
Using the daylight, lamps, signs, and other light already at a location instead of building a formal lighting setup.
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What it is
Available light is the light already in the location before a formal photo setup arrives: daylight through a window, ceiling fluorescents, streetlights, candles, or a shop sign. Natural light is the sun and sky part of that category. The terms overlap in casual use, but an existing tungsten lamp is available light and not natural light.
Reach for it when the location should still feel like itself, when a flash would disturb people, or when speed matters more than total control. Move the subject nearer a window, turn off ugly ceiling lights, open a curtain, or use a white wall as bounce. Working with available light still involves choices; you are arranging the scene around the sources instead of arranging the sources around the scene.
Gotcha: mixed sources can paint different parts of one face different colors, and no single white-balance setting can make blue daylight and orange tungsten both neutral. Low light also tempts slow shutter speeds that blur people. Pick the source whose color you want to keep, remove the others when possible, and raise ISO before sacrificing a shutter speed the subject cannot hold.
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Photograph this with available light only: use the north-facing window as a large soft key, switch off the overhead tungsten fixtures, and place a white foamcore bounce opposite the window. Set white balance to Daylight, expose at 1/125 second or faster to hold the person still, and allow ISO to rise before slowing the shutter. Add no flash, LED panel, studio strobe, or invented second light direction.