Symbiosis

• The living together in intimate association of two dissimllar organisms, so that the cohabitation is mutually beneficial.  
• association of two different species of organisms for mutual benefit, living together
  Source: Noland, George B. 1983. General Biology, 11th Edition. St. Louis, MO. C. V. Mosby
• The close association of two or more dissimilar organisms where both receive an advantage from the association.