Bava Batra 55B

ודרך הרבים, ודרך היחיד, ושביל הרבים, ושביל היחיד הקבוע בין בימות החמה ובין בימות הגשמים.

and a public road that is at least 16 cubits wide; and a private road that is 4 cubits wide; and a public trail; and a permanent private trail that is used whether in the summer or in the winter, i.e., winter. Rav Asi and Ravin disagree with regard to whether R' Yoḥanan held that a boundary or sea squill also serves to subdivide a field for the purpose of pe’a.

טומאה ⁠– מאי היא? דתנן: הנכנס לבקעה בימות הגשמים, וטומאה בשדה פלונית, ואמר: הלכתי למקום הלז, ואיני יודע אם נכנסתי לאותו מקום ואם לאו ⁠– רבי אליעזר מטהר, וחכמים מטמאין.

The Talmud further clarifies: What is the halakha of ritual impurity that is affected by determining whether an area is one or two fields? As we learned in a Mishnah (Teharot 6:5): With regard to one who enters into a valley during the winter, i.e., winter, when people generally do not enter this area, and therefore for the purpose of this halakha it is considered a private domain, and there is a principle that in a case of uncertainty concerning whether one contracted ritual impurity in a private domain he is ritually impure; and there was ritual impurity in such and such a field, and he said: I know I walked to that place, i.e., I walked in the valley, but I do not know whether I entered that place where the ritual impurity was or whether I did not enter, R' Eliezer deems him pure and the Rabbis deem him impure.

שהיה רבי אליעזר אומר: ספק ביאה ⁠– טהור, ספק מגע טומאה ⁠– טמא.

R' Eliezer deems him pure, as R' Eliezer would say: Concerning uncertainty with regard to entry, i.e., it is uncertain whether he entered the area where the ritual impurity is located, he is ritually pure. But if he certainly entered the area where the ritual impurity is located and the uncertainty is with regard to contact with ritual impurity, he is ritually impure. It is with regard to this halakha that Ravin said in the name of R' Yoḥanan that a boundary or sea squill defines these fields as distinct areas.