Fluid (from Latin fluidus – fluent) – any substance, which behavior may be described by fluid mechanics laws. This scientific term was introduced in the 17th century to denote hypothetical liquids used to explain certain chemical phenomena and rock formation. In the process of science development the content of the concept of “fluid” changed. Rheogoniometric and geological studies proved that all real bodies – no matter how hard they may seem – behave like liquids under long-lasting circumferential loads. If the action time (t) of external force causing tangential stress is significantly shorter than the relaxation time (tr), then the body demonstrates elastic behavior. If t>tr, the body behaves like liquid, i. e. flows. In geological process, which are often measured in millions of years, various substances may perform as fluids – gases, aqueous solutions, oil, slurries, magma, clays, salts, gypsums, anhydrates, limestone and other solid materials.