Aperture in PCB industry has 3 main definitions:

1. An indexed shape with a specified x and y dimension, or line-type with a specified width, used as a basic element or object by a photo plotter in plotting geometric patterns on film. The index of the aperture is its Position (a number used in an aperture list to identify an aperture) or D code.

2. A small, thin, trapezoidal piece of plastic used to limit and shape a light source for plotting light patterns on artwork film, and mounted in a mechanical disk called an “aperture wheel” which in turn is mounted on the lamp head of a vector photoplotter. An aperture is mostly opaque, but with a transparent portion that controls the size and shape of the light pattern. A vector photo plotter plots images from a CAD database on photographic film in a darkroom by drawing each line with a continuous lamp shined through an annular-ring aperture, and creating each shape (or pad) by flashing the lamp through a specially sized and shaped aperture.

3. A line of textual data in an aperture list describing the index names (D code and position), the shape, the usage (flash or draw) and the X and Y dimensions of an aperture. Some aperture lists leave out certain of those types of data. For example, laser photoplotters don’t need to know whether an aperture is a flash or draw, so a modern-day aperture list might leave that datum out.