For example, suppose we are in the key of G major. In this key the major triad A - C# - E is non-diatonic, en the tone C# forms an alteration or altered tone.  However, this triad is well placeable in other keys, for example as I in A major, VI in F# minor, V in D major, etc. This means that it is not an altered chord. And in the key of G major the triad A - C# - E probably is a secondary dominant:
(V) --> V
 

See also example 2. 
 

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