Om Namah Shivaya (ॐ नमः शिवाय ) & Maha Mritunjay Mantra ( महामृत्युंजय मंत्र ) (a.k.a Rudra Mantra or Tryambakam Mantra) or Great Death-conquering Mantra, are one of the most important ,powerful and popular Hindu Mantra. Namah Shivaya means "O salutations to the auspicious one!", or “adoration to Lord Shiva". It is called Siva Panchakshara, or Shiva Panchakshara or simply Panchakshara meaning the "five-syllable" mantra (viz., excluding the Om) and is dedicated to Lord Shiva. It is a holy salutation to Lord Shiva.
Maha Mritunjay Mantra - Aum Tryambakam Yajamahe Sugandhi Pushtivardhanam Urvarukmiv Bandhanan MrityoMurkshiya Maamratat (ॐ त्र्यम्बकं यजामहे सुगन्धिं पुष्टिवर्धनम् उर्वारुकमिव बन्धनान्मृत्योर्मुक्षीय माऽमृतात् )

In Siddha Shaivism and Shaiva Siddhanta Shaivism tradition Namah Shivaya is considered as Pancha Bodha Tatva of Lord Shiva and his universal oneness of five elements:

Na sound represents earth
Ma sound represents water
Śi sound represents fire
Vā sound represents Pranic air
Ya sound represents sky or ether
Its total meaning is that "universal consciousness is one" .


In the Shaiva Siddhanta Shaivism tradition the five letters also represents :

Na is the Lord’s concealing grace
Ma is the world
Śi stands for Shiva
Vā is His revealing grace
Ya is the Ātman or soul


Word-by-word Meaning of the Maha Mrityunjaya Mantra

ॐ aum = is a sacred/mystical syllable in Hinduism, Jainism, Buddhism & Sikhism.

त्र्यम्बकं tryambakam = to three-eyed one
त्रि + अम्बकम् = tri + ambakam = three + eye

यजामहे yajāmahe = in yagya in worship, (locative case)
सुगन्धिम् sugandhim = to fragrance, (accusative case),

पुष्टि puṣṭi = nourishment, sustenance
वर्धनम् vardhanam = increasing, flourishing
पुष्टि-वर्धनम् = puṣṭi+vardhanam = nourishment-increasing ( compound word)

उर्वारुकमिव urvārukam-iva = cucumber
बन्धनान् bandhanān = "from bondage {i.e. from the stem of cucumber} (of the gourd)
bandhanāt means from bondage Thus, read with urvārukam iva, it means 'as cucumber from bondage ( of vine) (to a vine)'

मृत्योर्मुक्षीय mṛtyormukṣīya = liberate from death
मृत्योः + मुक्षीय = mṛtyoḥ + mukṣīya= from death + free (Vedic usage)
माऽमृतात् अमृतात् = amṛtāt = by amrita, by immortality