In photography, "aperture" refers to the opening in the lens diaphragm. The diaphragm is a thin multi-bladed plane with an opening in it (the aperture) situated at the point in a lens where light converges and inverts. Adjusting the blades allows the aperture to be opened or closed at specific settings, called stops.
Aperture is one of the key elements in the exposure triad. It controls how much light passes through the lens and reaches the imaging plane. Aperture is also the key element of control over depth of field in a shot, as tighter apertures (smaller opening) allow for greater depth than wider apertures (bigger opening). Aperture is usually rated in stops, where each stop is a difference of twice the area of the opening.
The aperture of a lens rated in "relative" aperture values, such as f/2.8, f/5.6, and so on. Lens descriptions usually refer to the maximum aperture of the lens. Lenses also have a minimum aperture, which varies between lenses, but is usually somewhere between f/22 and f/64, with a very common minimum aperture being f/32. The maximum aperture of a lens is often also used to indicate a lenses "speed". Wider maximum apertures are "faster" lenses, as they allow more light to pass at any given moment. Tighter maximum apertures are "slower" lenses, as they allow less light to pass at any given moment.