Frequency bands are categories
EEG records electrical activity from the scalp and researchers commonly analyze activity across frequency ranges. Boundaries can vary by convention and context, and multiple patterns can coexist.
Delta
Delta refers to relatively slow activity and is commonly discussed in relation to deep sleep research.
Theta
Theta activity appears in several cognitive and sleep contexts. Commercial audio frequently associates theta with meditation, but a frequency label alone does not guarantee a meditative state.
Alpha
Alpha rhythms are often studied during relaxed wakefulness and attention. Their interpretation depends on brain region, task and measurement context.
Beta and gamma
Faster activity is often analyzed in relation to active cognitive processing, but simple “higher frequency equals better cognition” claims are misleading.
For the comparison people commonly make, read alpha vs theta. For auditory methods, see brainwave entrainment.