A phosphite is a salt of phosphorous acid. The phosphite ion (PO33−) is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3. Its molecular geometry is trigonal pyramidal like ammonia.
Because phosphorous acid exists as an equilibrium tautomeric mixture of P(OH)3 and HP(O)(OH)2, predominantly the latter, there is some confusion in nomenclature. The IUPAC recommends that the trihydroxy form be called phosphorous acid and its salts phosphites, with the dihydroxy form being called phosphonic acid and its salts phosphonates, but despite this, salts of HP(O)(OH)2 are often called phosphites rather than phosphonates.
The term phosphite is also used to mean phosphite ester, an organophosphorus compound with the formula P(OR)3.
Read more about Phosphite: Acid Phosphites, Pyrophosphites, Naming of Phosphite Ions, Synthesis of Phosphite Esters, Use in Plants