What is Bill To Perpetuate Testimony?

Legal Definition
A bill in equity filed in order to procure the testimony of witnesses to be taken as to some matter not at the time before the courts, but which is likely at some future time to be in litigation. Story, Eq. Pi. (5th Ed.) § 300 et seq.
-- Black's Law Dictionary
Legal Definition
A bill in equity to secure and preserve testimony for use in an action not yet pending. See 1 Del. Ch. 133.
-- Ballentine's Law Dictionary