dead pixel (contrast hot pixel)

This is more important in Astrophotography where it is necessary to take a dark field exposure (an image with the lens cap on) and use this to correct (by subtraction) for any places where the sensor registers although no photons were received.  These are "hot pixels" and would yield false stars in astrophotography.  The opposite of a "hot pixel" is a "dead pixel" which records no value, even when exposed to white light.  Dead pixels can be caused by dust on the sensor, but in time pixels may fail on a sensor.  In ordinary photography these will show up as black or white spots when zoomed in to actual pixels (or more).  If these occur in high contrast situations; e.g. a dead pixel in a bright field or a hot pixel in a dark field they may show up in which case you can clone them out, using a neighbouring mid tone or appropriate colour pixel.

DNG (Digital NeGative)

An open standard format for RAW files.   It was developed by Adobe in 2004 and they then submitted it to ISO for consideration to be issued as a standard.   One of the problems with technology is that standards can be rapidly out dated and superceded (remember the optical disk and BetaMax) which is a serious problem for archivists.   One of the design objectives of the DNG standard was archival purposes.

dodge   c.f. burn.

DPI (Dots Per Inch)

These are only relevant to printing and will not be discussed further.

DSLR (Digital Single Lens Reflex.)

Like a film SLR but using a digital sensor instead of a film.   Originally these were SLR cameras with mirrors and focal plane shutters, now much more is done electronically.   The term single lens reflex contrasts with the Rolliflex twin lens reflex which sadly is just a memory now.   See also Bridge Camera.