Stearic Acid (also known as Stearin or Stearine) is a wax additive for paraffin wax.  It is used to harden wax and help it to shrink while cooling for better mould release, and it will also make the wax more opaque.  It is recommended for use in votive and pillar candles, and should not be used with silicone and polyurethane moulds as it will corrode the mould (Vybar should be used instead).  Recommended usage is 10% although this can be increased by 2-3% when using heavier fragrances to avoid fragrance seepage, and can be helpful with fragrance oils which don't absorb well.  Stearic Acid can be used on its own to make wax embeds and also pure Stearic candles.

MSDS