CPCTC is short for
It is intended as an easy way to remember that when you have two
and you have proved they are
then each part of one triangle (side, or angle) is congruent to the corresponding part in the other.
For more on congruent triangles, see Congruent triangles - introduction
Many people find it actually easier to remember the fact than the letters, because the letter sequence is easy to get wrong from memory.
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