Definition of Interplea:
“(17c) A pleading by which a stakeholder places the disputed property into the court’s registry; the plea made by an interpleader.”
Definition of Interplead:
“vb. (16c) 1. (Of a claimant) to assert one’s own claim regarding property or an issue already before the court. 2. (Of a stakeholder) to institute an interpleader action, usu. by depositing disputed property into the court’s registry to abide the court’s decision about who is entitled to the property.”
Definition of Interpleader:
“n. (16c) l. A suit to determine a right to property held by a usu. disinterested third party (called a stakeholder) who is in doubt about ownership and who therefore deposits the property with the court to permit interested parties to litigate ownership. Typically, a stakeholder initiates an interpleader both to determine who should receive the property and to avoid multiple liability. 2. Loosely, a party who interpleads. — Also termed (in civil law) concursus.” 
Excerpt from Charles Alan Wright’s The Law of Federal Courts:
“Interpleader is a form of joinder open to one who does not know to which of several claimants he or she is liable, if liable at all. It permits him or her to bring the claimants into a single action, and to require them to litigate among themselves to determine which, if any, has a valid claim. Although the earliest records of a procedure similar to interpleader were at common law, it soon became an equitable rather than a legal procedure.” 
Definition of interpcIIatio:
“n. [Latin “a demand, interruption”) Roman law. 1. A demand for payment of a debt or for desistance from a course of action. 2. The interruption of a process, e.g., of the acquisition of title by possession. 3. The institution of a legal process or appeal. Pl. interpcllationes” 
Download a PDF of the Official Form from UScourts.gov:
Complaint for Interpleader and Declaratory Relief
(this same form can be downloaded from UScourts.gov here)
Transcript of Rule 22:
(1) By a Plaintiff. Persons with claims that may expose a plaintiff to double or multiple liability may be joined as defendants and required to interplead. Joinder for interpleader is proper even though:
(A) the claims of the several claimants, or the titles on which their claims depend, lack a common origin or are adverse and independent rather than identical; or
(B) the plaintiff denies liability in whole or in part to any or all of the claimants.
(2) By a Defendant. A defendant exposed to similar liability may seek interpleader through a crossclaim or counterclaim.
(b) Relation to Other Rules and Statutes. This rule supplements—and does not limit—the joinder of parties allowed by Rule 20. The remedy this rule provides is in addition to—and does not supersede or limit—the remedy provided by 28 U.S.C. §§1335, 1397, and 2361. An action under those statutes must be conducted under these rules.
: All definitions from: Black’s Law Dictionary Deluxe Tenth Edition by Henry Campbell Black & Editor in Chief Bryan A. Garner. ISBN: 978-0-314-62130-6
: Charles Alan Wright, The Law of Federal Courts 5 74, at 531 (5th ed. 1994).