Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.
WRCC Climate and Weather Terms Glossary

Fine dry or wet particles of dust, salt, or other impurities that can concentrate in a layer next to the Earth when air is stable. 

NASA Earth Observatory Haze - An atmospheric aerosol of sufficient concentration to be visible. The particles are so small that they cannot be seen individually but are still effective in visual range restriction. The Blue Ridge Mountains of the Eastern United States have a haze caused by the natural emissions of hydrocarbons (mostly isoprene-see below) which are oxidized and form aerosols. [Graedel, T.D. and Paul Crutzen. Atmospheric Change: an Earth system perspective; 1993; Freeman Press.]  Atmospheric Chemistry Glossary