An Auditory Processing Disorder (APD) is a disruption in the auditory nervous system that interferes with the rapid and efficient awareness, recognition, decoding and integration of acoustic signals, especially speech.

An auditory processing disorder may be considered a  breakdown in a child's auditory fine resolution abilities.  By fine resolution I imply the hearing system's ability to discretely resolve the fine differences that we use to locate, separate, and distinguish sounds in the environment.  In terms of speech perception, there are many fine grained distinctions needed to successfully distinguish the many sounds that are embedded in the ongoing stream of running speech.  For instance, recognizing the differences between the sounds in the words [bat] and [mommy] is not too demanding.  However, distinguishing the sounds in the words [ran] and [ram] or [lift] and [list] are more difficult tasks.  The latter tasks require a more efficient auditory system to rapidly identify and distinguish each pair's sounds than it does to distinguish between the sounds in the first pair.  This relates to rapidly resolving the small and fine grained speech sound distinctions. 

Children with APD also have problems resolving other acoustic properties, such as time variations in speech.  The two words [dime] and [time] are distinguished, predominantly, by the initiation of vocal fold vibration.  The vocal folds begin to vibrate earlier when producing the [d] in dime then when producing the [t] in time.  A child who does not resolve fine time differences may have difficulty distinguishing these two words.  Timing is also important in 'juncture'.  Juncture relates to the way we separate words with spaces (or time) when speaking.  Examples of varying junction include: 1) "Are you a light housekeeper?"  vs. "Are you a lighthouse keeper?"  2) "Look at the snow, drift by the window." vs. "Look at the snowdrift, by the window."  3) "Look at the cargo." vs. "Look at the car go."