Registrant
The person or organization that holds the registration for a domain. Identified by the registrant contact in WHOIS (now usually redacted under GDPR).
Vs admin / tech contact
The registrant is the legal holder. Admin contact handles registration changes; tech contact handles DNS issues. All three can be the same person and usually are for individual registrations.
Ownership transfers
Changing the registrant inside the same registrar is a Change of Registrant (COR) and triggers a 60-day transfer lock unless both parties opt out.