Legal definition of infringement
Copyright Legal Terminology

“It is of course essential to any protection of literary property, whether at common-law or under the statute, that the right cannot be limited literally to the text, else a plagiarist would escape by immaterial variations. That has never been the law, but, as soon as literal appropriation ceases to be the test, the whole matter is necessarily at large, so that, as was recently well said by a distinguished judge, the decisions cannot help much in a new case. … When plays are concerned, the plagiarist may excise a separate scene; … or he may appropriate part of the dialogue …. Then the question is whether the part so taken is “substantial,” and therefore not a “fair use” of the copyrighted work; it is the same question as arises in the case of any other copyrighted work. ….
But when the plagiarist does not take out a block in situ, but an abstract of the whole, decision is more troublesome. Upon any work, and especially upon a play, a great number of patterns of increasing generality will fit equally well, as more and more of the incident is left out. The last may perhaps be no more than the most general statement of what the play is about, and at times might consist only of its title; but there is a point in this series of abstractions where they are no longer protected, since otherwise the playwright could prevent the use of his “ideas,” to which, apart from their expression, his property is never extended. …. Nobody has ever been able to fix that boundary, and nobody ever can.”