"Halachah" - (f., pl. "Halachot");
This word is used in two senses: one is as a singular, meaning a single Jewish Law; for example, that one has to recite a "Berachah, a blessing, before one eats an apple.
The second sense is as a collective noun, referring to "all of Jewish Law." For example, a Rabbi should be an expert in all of "Halachah."
The word derives from the Hebrew "shoresh," or root, "lech," "to go." The reason is that it describes a "path to follow," or a "way on which to go," in life Judaism 101