The Shiva Linga or Lingam is a symbol that represents Lord Shiva in Hinduism. As the most powerful of deities, temples are built in his honor that includes a Shiva Linga, representing all the energies of the world and beyond. 

Seshnag is a large thousand headed serpent often referred to as the king of all serpents in the Hindu mythology. It is said that all the planets of the universe are located on its heads and each time he shifts Earth from one head to another an earthquake takes place. It is said that when he uncoils, time moves forward and creation takes place. When he coils back, the universe ceases to exist.
The popular belief is that the Shiva Linga represents the phallus, the emblem of the generative power in nature. According to followers of Hinduism, their teachers have taught that this is not only a mistake, but also a grave blunder. Such a stance, for example, can be found in the teachings of Swami Sivananda,In addition to the Hindu tradition, the Shiva Linga has been adopted by a number of metaphysical disciplines. In this case, it refers to a particular stone from an Indian river that is believed to have healing powers for the mind, body, and soul.

To understand these dual uses for the words Shiva Linga, let's approach them one at a time and begin with the origin. They are completely different but connected in their underlying meaning and connection to Lord Shiva.