The Mahamrityunjaya Mantra

ॐ त्र्यम्बकम् यजामहे सुगन्धिम् पुष्टिवर्धनम् ।

          उर्वारुकमिव बन्धनान् मृत्योर्मुक्षीय माम्रतात् ।।
In English :

aum tryambakam yajāmahe sugandhim pusti-vardhanam

           urvārukam iva bandhanān mrtyor muksīya māmrtāt

The Mahamritryunjaya Mantra was found by Rishi Markandya. It was a secret Mantra, and Rishi Markandya was the only one in the world who knew this Mantra. Once the Moon was in trouble when it was cursed by Prajapati Daksh. Then Rishi Markandey Gave the Mahamritryunjay Mantra to SATI for the Moon. This is why we know this mantra very well. Along with the Gayatri mantra it is one of the most widely known mantras of contemporary Hinduism.
 This great mantra dedicated to Rudra as Mrityunjaya is found in the Rig Veda. It is called the Maha Mrityunjaya mantra, the Great Death-Conquering mantra. It is a mantra that has many names and forms. It is called the Rudra mantra, referring to the furious aspect of Shiva; the Tryambakam mantra, alluding to Shiva's three eyes; and it is sometimes known as the Mrita-Sanjivini mantra because it is a component of the "life-restoring" practice given to the primordial sage Shukra after he had completed an exhausting period of austerity. The Maha Mrityunjaya mantra is hailed by the sages as the heart of the Veda. Along with the Gayatri mantra it holds the highest place among the many mantras used for contemplation and meditation
Word to Word Meaning of Maha Mrityunjaya Mantra

• ॐ aum = is a sacred/mystical syllable in Hindu Religion and is mostly used in beginning of all Mantras
• Tryambakam = the three-eyed one (accusative case)
• Yajāmahe = We worship, adore, honor, revere

• Sugandhim = sweet smelling, fragrant (accusative case)
• Pusṭi = A well-nourished condition, thriving, prosperous, fullness of life
• Vardhanam = One who nourishes, strengthens, causes to increase (in health, wealth, well-being); who gladdens, exhilarates, and restores health; a good gardener

• Urvārukam = pumpkin (in the accusative case)
• iva = like, just as

urvārukam: 'urva' means "vishal" or big and powerful or deadly. 'arukam' means 'disease'. Thus urvārukam means deadly and overpowering diseases.  

• bandhanān = "from captivity" {i.e. from the stem of the cucumber} (of the gourd); (the ending is actually long a then -d which changes to n/anusvara because of sandhi)

bandhanān means bound down. Thus read with urvārukam iva, it means 'I am bound down just as by deadly and overpowering diseases'.

• mṛtyor = From death
• mukṣīya= Free us, liberate us
• mā = not
• amṛtāt = [from] immortality, emancipation

Simple Translation

We hail the fragrant Three-eyed One who nourishes [all] and increases the [sweet] fullness of life. As the cucumber is liberated from captivity [from its stem], may we [also] be liberated (mukshiya) from death (mrityor) not from immortality (maamritaat).

Actually OM is not spelled out in the Rig-Veda, but has to be added to the beginning of all Mantras as given in an earlier Mantra of the Rig-Veda addressed to Ganapati.

According to some puranas, the mahamrutyunjaya mantra has been used by many rishis as well as Sati during the time when Chandra suffered from the curse of Prajapati Daksha. By reciting this mantra, the effect of the curse of daksha, that could make him die, slowed, and Shiva then took Chandrama on his head.