Standard Sea Level (also known as Sea Level Standard (SLS)) defines a set of conditions for physical calculations.
The term standard sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 is used to indicate that values of properties are to be taken to be the same as those standard at sea level, and is done to define values for use in general calculations.

Atmospheric properties

At SSL some atmospheric properties are:

• Pressure, P = 101,300 Pa
  Pascal (unit)
  The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
  
   = 2116 lbf
  Pound-force
  The pound force is a unit of force in some systems of measurement including English engineering units and British gravitational units.- Definitions :...
  
  /ft²
  Square foot
  The square foot is an imperial unit and U.S. customary unit of area, used mainly in the United States, Canada, United Kingdom, Hong Kong, Bangladesh, India, Pakistan and Afghanistan. It is defined as the area of a square with sides of 1 foot in length...
  
  
• Density, = 1.225 kg/m³ = 0.07647 lb
  Pound (mass)
  The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...
  
  /ft³ = 0.002377  slug/ft³
• Temperature, T = 288.16 K
  Kelvin
  The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
  
   = 518.69 °R
• Gas constant
  Gas constant
  The gas constant is a physical constant which is featured in many fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation. It is equivalent to the Boltzmann constant, but expressed in units of energy The gas constant (also known as the molar, universal,...
  
   of air, R_air = 286.97 m²/(s²·K) = 1716 ft²/(s²·°R)
• Dynamic viscosity
  Viscosity
  Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
  
  , = 1.79×10⁻⁵ Pa·s = 3.737×10⁻⁷ slug/s·ft

Sources

• Aerodynamics, aeronautics, and flight mechanics
  Aeronautics
  Aeronautics is the science involved with the study, design, and manufacturing of airflight-capable machines, or the techniques of operating aircraft and rocketry within the atmosphere...
  
  by Barnes Warnock McCormick Edition: illustrated Published by Wiley, 1979 Original from the University of Michigan Digitized Dec 14, 2007 ISBN 0-471-03032-5, 978-0-471-03032-4 652 pages