Rashi on Exodus 28:15/רש"י על שמות כ״ח:ט״ו

1חשן משפט THE BREAST-PLATE OF JUDGMENT — it was so called because it atoned for any error in judgment (cf. Zevachim 88b; Jerusalem Talmud Yoma 7:5). Another explanation of why it is called “the breast-plate of משפט”, is because it makes its statements clear (definite — so that there can be no doubt about their meaning) and its promise is true; discerement in old French, (sentence of judgment). Because the word משפט is used in three senses: the contentions of the parties to the law-suit, the pronouncement of the sentence and the punishment attendant on the sentence, whether it be punishment of death or punishment of flagellation or punishment involving a monetary payment; here it is used in the sense of making its statements definite, so that this is the breast-plate with the Urim and Thummim which explains and makes its statement clear.

2כמעשה אפד AFTER THE WORK OF THE EPHOD — the work of an artist and of the five materials.