A guru is a realized teacher who can guide the sincere aspirant. The word is derived from the root ‘gru’ which means heavy or weighty. Another popular etymology given in Hindu scripture is ‘gu’, darkness; ‘ru’, remover; thus dispeller of darkness.

Guru is also one of the names of the planet Jupiter (Brihaspati in Sanskrit). In Jyotish and mainstream Hindu cosmology, Brihaspati is said to be the spiritual preceptor or teacher of the Devas (gods).

Concerning the Guru, Shri Gurudev Mahendranath once wrote,

“The Guru is often symbolized by a flame, the light of a lamp. The light is intended to light up the scene and to turn the dark situation into one filled with brightness and clarity. This flame is not the situation, nor does it tell you which way to go. Thus in the dark zones of the mind and the confusing labyrinth of the occult world a reliable lamp can turn obscurity into brilliant illumination.”

Further reading
Practical Guru Guidance by Sri Kapilnath