Compartmentation (or compartmentalisation) is the practice of keeping different identities, tools, and activities walled off from one another — separate usernames, emails, browsers, and devices — so that no shared detail can tie them together. It is the single most powerful privacy and OPSEC technique.
For an investigator it protects the research and the researcher; for a privacy-conscious individual it prevents a pseudonymous persona from being linked to their real identity.