noun

• (physics, meteorology, astronomy, optics) The fraction of incident light or radiation reflected by a surface or body, commonly expressed as a percentage.
  "For track, painting rails white at weak points that are more prone to buckling (where the 'critical rail temperature' is close to the 'stress free temperature' [...]) increases their albedo and can help slow how quickly they heat up, reducing their temperature by perhaps five degrees or so."
• (botany) The whitish inner portion of the rind of citrus fruits that is a source of pectin, commonly referred to as the pith.
• (alchemy) One of the four major stages of the magnum opus, involving purification of the prima materia.