In order for readers to better understand some of the complexities that go into building and maintaining an eruv, we are including a glossary of terms. To those familiar with the laws of eruvin, these terms speak volumes. There is much that goes into ensuring an eruv complies with halacha - here are some of the basics:
RESHUS: This refers to any of the four common domains into which property can be divided.
These primary domains are:
1) Reshus hayachid, A private domain - A place set aside from its surroundings by any of a number of devices such as a wall, a series of doorway shaped enclosures or even by a steeply pitched slope leading up to the reshus.
2) Reshus harabim, A public domain - A large, publicly used thoroughfare. The classic example is the travels of the B'nai Yisrael in the desert. To that end, halacha dictates it be similar to those pathways. There are guidelines relating to minimum width (16 amos - approximately 24 to 32 feet) and number of passersby, (600,000 - like the B'nai Yisrael.) The applications of each of these are subject to major dispute among the Rishonim.
3) Karmelis - A large, open area which lacks the characteristics of both a Reshus hayachid and a Reshus harabim and does not have the remediation of a reshus hayachid.
4) A Makom p'tur- place which lacks the basic necessities required to be considered an area of "common use", and is held to be almost halachiclly transparent.
GEDER: A wall or other solid enclosure which defines the boundaries of a reshus hayachid.
TZURAS HAPESACH: a structure shaped like a doorway. This can take the place of a full wall, and may constitute a side (or parts of a side ) of the enclosure necessary to make a location a Reshus Hayachid
LECHI: A vertically positioned pole which acts as one member of the "doorway" in the Tzuras Hapesach. This lechi must be positioned vertically and directly beneath the koreh elyon.
KOREH ELYON: The upper, horizontal element which completes the "doorway" shape which composes a wall of the eruv enclosure. This koreh may consist of a wire, a pole or any other means of connecting the two vertical lechayayim (vertical poles). It is critical that this koreh pass directly over the lechi.
PIRTZAH: A break in the enclosure disqualifies the "wall" effect if it is greater than 10 amos (18 to 24 feet.)
LAVUD: This is a concept handed down to Moshe Rabbaynu at Har Sinai, which states that structures within three t'fachim (9 to 12 inches) of one another are seen as if they were touching. Hence if a wall is not directly in contact with its adjacent structure, it is none-the-less seen as enclosed. This concept, too, will mean that the lechi need not directly be in contact with the koreh--as long as the lechi is directly below the koreh it is sufficient to be considered touching
GUD ASIK: Another Halacha L'Moshe M'Sinai, this legal mechanisim states that we may theoretically extend certain surfaces or items to consider enclosures finished, even when the various elements are not in actual physical contact. This concept, too, will mean that the lechi need not directly be in contact with the koreh--as long as the lechi is directly below the koreh it is sufficient to be considered touching. This halacha also had other applications, such as extending imaginary lines of walls both upwards and downwards.
TEL HAMISLAKET: One of the ways a reshus hayachid is demarked is by a sharply rising slope, one which reaches the height of 10 tfachim (30 to 40 inches) within the span of 4 amos (6 to 8 feet.)
HASKARAS RESHUS: As a prelude to encircling the "eruv" area, the entire "reshus" needs to be consolidated into one homogeneous property. This is accomplished by technically "leasing" the entire property from a representative of the state government. Though the transaction involves private properties, the principle of Eminent Domain allows for this transaction.
KARFIF: A large area that is not traversable and is not meant to be used in a common everyday manner. Such an area cannot be included in an area defined by an eruv. If a karfif exists within an eruv, it must be excluded by encircling it with its own walls or a series of tzuras hapesachs.