When Rashi uses the synonym method he does not explain the meaning of a word but rather the distinction between two similar words both of whose meanings we already know.

The following Hebrew words all refer to laws.
• Mem-Shin-Pay-Tet, MishPat: civil law;
• Daleth-yud nun, Din: court law / ruling;
• Mem-Tzade-Vav-Hey, Miztvah: religious law;
• Cheth-Kuph, Chok: absolute / statutory law;

In our article Peshat and Derash: A New Intuitive and Logical Approach, which can be found on the world-wide-web at http://www.Rashiyomi.com/rashi.pdf we have advocated punchy translations of Biblical verses as a means of presenting Rashi comments. The following translation of verse Gn47-25c:26 embeds the Rashi translation Chok means an absolute law. And they said, You have saved our lives; let us find grace in the sight of my lord, and we will be Pharaoh’s servants [to pay an annual 20% tax under all circumstances] And Joseph made it an absolute / statutory law over the land of Egypt to this day, that Pharaoh should have the fifth part; except the land of the priests only, which became not Pharaoh’s.

Advanced Rashi: The difference between an ordinary law and an absolute law is that ordinary laws can be situational. For example, if the 20% tax was an ordinary law then it could be waived under circumstances of poverty or expensive illness. By making the law absolute Joseph assured the monarchy a 20% cut of all profit in Egypt.

Many people erroneously interpret Chok to mean a law without reason. Rabbi Hirsch shows this is an incorrect approach. His best proof is the verse Pr30-08 Remove far from me falsehood and lies; do not give me poverty nor riches; hunt for me my absolutely needed food supply. Rav Hirsch points out The word Chok in this verse does not refer to an irrationally decreed food amount but rather to the person's minimal absolutely needed food amount. Chok in general refers to a law that addresses a deep seated need that does not change.

I have often explained Rav Hirsch by using an analogy of poison vs. salt. If you consume poison you die immediately. But if you consume excess salt you will not see deleterious effects for a long while. Thus the prohibition of salt is a statutory prohibition, whose reason is not apparent now but becomes apparent over a long period of time. Using this analogy I explain the Talmudic statement:A Chok is law that the non-Jewish nations make fun of us and ask "Why do you observe these laws?" The point of this Talmudic statement, cited by Rashi, is not that Chok is without reason but rather that its reason is not immediately apparent and only manifests itself over time. In fact this Talmudic dictum rather than questioning the rationality of the chukim is instead questioning the ability of non-Jewish mockers for long-term goals.

The bottom line is that Chok refers to an absolute law based on a reason that is not apparent and will manifest only after a long period. The Chok unlike other laws is less subject to exceptional circumstances.