Medscape defines diffuse as generalized, in the context of EEG. Generalized means activity recorded across large portions of the cortex. This opposes focal patterns, that occur locally. In turn this is reflected in generalized epilepsy and focal epilepsy. Generalized epilepsias are characterized by gross paroxysmal activity across the cortex, associated with a loss of consciousness. Focal epilepsy is localized in the cortex and stays restricted to one hemisphere and is not associated with a loss of consciousness.
Britton et al. (2016) explain generalized and focal slowing in the EEG as follows:
The two main types of slowing are focal and generalized slowing. As
previously discussed, generalized background slowing in the theta and
delta frequency ranges is a normal finding on EEG when it represents
developmental slowing  or the evolution of drowsiness and sleep activity. However, when there is intermittent or persistent focal slowing seen consistently over one head region, or persistent, unvarying, and unreactive focal or generalized slow wave activity in a vigilant adult patient, this slow wave activity should be considered pathologic and indicates
corresponding focal or generalized cerebral dysfunction or both.
- Britton et al. The Abnormal EEG. In: St. Louis & Frey (eds). Electroencephalography (EEG): An Introductory Text and Atlas of Normal and Abnormal Findings in Adults, Children, and Infants. Chicago: American Epilepsy Society; 2016