"Waheguru"

Waheguru ji ka Khalsa, Waheguru ji ki Fateh !

The word 'Waheguru' liertally means the "Wonderful Lord" in the Gurmukhi language.
Waheguru is the Gurmantra or primary Mantra; it is the reference to the Almighty God; the Creator; the Supreme Soul; the Sustainer; etc.
God has many names in Sikhism and Waheguru is probably the most important and the most common.
Waheguru vahiguru or vahguru is a term most often used in Sikhism to refer to God, the Supreme Being or the creator of all.
It means "The Wonderful Teacher" in the Punjabi language.
"Wah" translates to "wonder" and "Guru" is a term denoting "teacher".

The most common usage of the word Waheguru is in the greeting Sikhs use with each other:
Waheguru Ji Ka Khalsa. can be described as "The Khalsa (or pure ones) belong to God"
Waheguru Ji Ki Fateh means "Victory belongs to God".

Cumulatively, the name implies wonder at the Divine Light eliminating spiritual darkness.
It might also imply, "Hail the Lord whose name eliminates spiritual darkness."

Considering the two constituents of "Vahiguru" (vahi + guru) implying the state of wondrous ecstasy and offering of homage to the Lord, the first one was brought into the devotional system by Guru Nanak.