From diadochokinesia, meaning the ability to bring a limb or other body part into opposite positions, i.e. flexion and extension or pronation and supination. It comes from the Greek Diadochos, meaning 'to work in turn', and kinesis, meaning 'movement'.
AKA Alternating Motion Rate (AMR)
Diadochokinetic rate (DDK rate) is a term used, to the best of my knowledge, only in the field of speech language pathology. As you might expect, within this field it is used to refer to the range of movement of the tongue and lips, generally when used to make speech sounds (Speech language pathologists (SLPs) also work with swallowing disorders, and DDK may be looked at in these cases too).
The measurement of DDK rate is a common component of a standard oral-motor exam, a brief checkup to make sure that there are no major problems in mouth function or structure. DDK rate measures the ability of the tongue to move from the position from one speech sound to another quickly. It is tested by having the subject say a set of sounds as quickly as possible; these sounds should show a wide range of motions of the tongue within the mouth, but by no means include all the sounds in the English language. In matter of fact, DDK will usually rely on one of a few stereotypical utterances; "puh-tuh-kuh" (which may broken into a set of "puh" followed by a set of "tuh", etc.), "pattycake", or "buttercup".
These utterances are chosen because they include sounds said at the front of the mouth (/p/ and /b/), the alveolar ridge (/t/), and the back of the mouth (/k/). A subject is usually asked to "say 'buttercup' ten times fast". At this point it is assumed that the therapist knows if the subject has any obvious speech problems such as a lisp or the inability to pronounce /r/s. This is only testing the speed that a subject can move their tongue to the positions needed to make sounds.
An inability to switch between sounds quickly may be indicative of apraxia, ataxia, dysarthria, and stuttering. Another related test used to check for these problems is the Maximum Repetition Rate (MRR), in which a single sound is repeated over and over again as fast as possible.
It is normal for children to have lower DDK rates than adults, but a marked inability to perform DDK and MRR tasks is one of the major diagnostic indicators of childhood apraxia of speech.
The full name of this test is "The Fletcher Time-by-Count Test of Diadochokinetic Syllable Rate". It is named after R.J. Fletcher, the researcher who first published DKK rate norms in 1972. These norms (how many times per second a person of a given age should be able to say a given sound) are still in use today. Some SLPs are starting to look at the accuracy and fluency of the sounds produced during the DDK in addition to the speed, though traditionally these are looked at through other forms of testing.