Mulnayak means the principal or main deity of a Jain temple. It is the central idol (usually a Tirthankar) that the temple is built around and to which the main daily worship and offerings are directed. The Mulnayak is considered the presiding spiritual power of that temple.

• In most Jain temples, the Mulnayak is a Tirthankar (for example, Mahavira or another Tirthankar in that line).
• There can be other revered idols in the temple (parshwanathas, subsidiary deities, etc.), but the Mulnayak holds the primary importance in worship and rituals.
• There are minor differences in emphasis between Digambar and Shwetambar traditions, but the core idea remains the same: the Mulnayak is the temple’s foremost spiritual focus.

If you’d like, I can share the exact wording and meanings of related Jain texts that discuss Mulnayak worship.