Bacterium that either does not support phage plaque formation or, more generally, successful phage infection.
Typically restrictive hosts do allow phage adsorption. Note, though, that blocks on phage propagation can occur by a variety of mechanisms and otherwise are not limited to host restriction-modification systems.
The "restriction" of restriction-modification, that is, is derived from the idea of a restrictive host rather than the idea of restrictive host following the discovery of restriction-modification systems. Indeed, restriction enzymes are named not because they "restrict" DNA but instead because they underlie one, prominent mechanism by which host can restrict phage propagation.
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