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CPCTC
CPCTC is short for
"Corresponding Parts of Congruent Triangles are Congruent"

It is intended as an easy way to remember that when you have two triangles and you have proved they are congruent, 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.