The Sanskrit Word 'Lingam' Means Symbol. Thus The  literal Meaning of Shiva Lingam is the symbol of shiva. The  Supreme Shiva doens't have a form and every form is his form. The Shiva Lingam represents him, The Supreme Shiva, who is formless. The way when we see a smoke, we infer the presence of fire, the moment we see Shiva Lingam we Immediately visualize the existence of the supreme Shiva.