In other words, by translating, rotating, and/or flipping one triangle, you can get it to coincide exactly with the other triangle.
If ΔABC is congruent to ΔDEF, then we write
In the figure below, ΔABC ΔDEF, but ΔABC is not congruent to ΔGHI.
Note also that we cannot write ΔABC ΔDFE. The corresponding vertices should come in the right order.