1936 W. G. Walter in 8 Aug. 308/1 It is suggested that..these slow waves from the neighbourhood of tumours be called ‘δ waves’ until their true nature be discovered.
1957 (ed. 23) 1540/2 Delta waves, waves in the electroencephalogram which have a frequency of ½ to 3 per second.
1968 24 202/1 The background consists of random slow activity which is symmetrical and there are some delta waves mixed with the alpha rhythm.