‘Only From Jericho And Below’
The Mishnah (54b), in reference to ma’aser, states that a person cannot treat all his flock as one unit if a geographic element separates them – e.g., the Jordan River.
On our daf, Rabbah b. Bar Chana says the Jordan River in this context means the part of the river starting from the city of Jericho and below. The Gemara notes, though, that in the context of vows, the Jordan River means the entire river in accordance with common parlance.
Meseches Nedarim (18b and 30a-31b) mentions many terms that restrict the force of a neder. The Gemara states that, as a rule, we define such terms in accordance with common parlance. The Rambam (Hilchos Nedarim 9:13) rules accordingly.
The Radbaz (to Rambam op. cit.) asks: Why do mishnayos in Nedarim define so many terms if we interpret all of them in accordance with common parlance in any event? The Radbaz answers that the definitions in these mishnayos are relevant in places where the terms carry no common meaning.
The Nimukei Yosef (ad loc., Nedarim 30b) cites the Ritva, who says that some definitions have changed since Mishnaic times. For example, the Mishnah (30b) states that if a person vows not to benefit from “shechorei ha’rosh (lit., dark-headed people), he is prohibited from benefiting from all adult men. Rashi explains that “dark-headed” does not include women since they always cover their heads.
The Ritva notes that nowadays “dark-headed” means people with dark hair. Thus, bald and older people (whose hair is usually white) are not included. He adds that even if the custom is to follow Mishnaic definitions, the common definition is adopted if the person making the vows specifies that the words he’s using should follow the common meaning – even if this stipulation produces a leniency.