What is Quasi?

Legal Definition
Latin for "as if." Commonly used as a prefix to show that one thing resembles, but is not actually, another thing. For example, a quasi-contract resembles, but is not actually, a contract.
Illustrative caselaw
See, e.g. Humphrey's Ex'r v. United States, 295 U.S. 602 (1935).
See also
Legal Definition
A Latin word in frequent use in the civil law signifying as if, almost. It marks the resemblance, and supposes a little difference between two objects. Dig. b. 11, t. 7, 1. 8, 1. Civilians use the expressions quasi-contractus, quasi-delictum, quasi-possessio quasi-traditio, &c.
-- Bouviers Law Dictionary
Legal Definition
Lat As if; as it were; analogous to. This term is used in legal phraseology to indicate that one subject resembles another, with which it is compared, in certain characteristics, but that there are also intrinsic differences between them. It is exclusively a term of classification. Prefixed to a term of Roman law, it implies that the conception to which it serves as an index is connected with the conception with which the comparison is instituted by a strong superficial analogy or resemblance. It negatives the notion of identity, but points out that the conceptions are sufficiently similar for one to be classed as the sequel to the other. Maine, Anc. Law, 332. Civilians use the expressions "quasi contractus," "quasi delictum," "quasi possessio," "quasi traditio," etc. As to quasi "Affinity," "Contract," "Corporation," "Crime," "Delict," "Deposit," "Derelict," "Easement," "Entall," "Fee," "In Rem," "Judicial," "Municipal Corporation," "Offence," "Pariners," "Personuity," "Possession," "Posthumous Child," "Purchase," "Rcalty," "Tenant," "Tort," "Traditio," "Trustee," and "Usufruct," see those titles.
-- Black's Law Dictionary
Legal Definition
"As if". Resembling or being similar to something, without actually being that thing.
Legal Definition
As; as if; as it were; relating to or having the character of. See 60 111. 390.
-- Ballentine's Law Dictionary