Full Idea
Entailment is the modern word saying that p logically follows from q. Its simplest definition is that you cannot have both p and not-q, but this has the problem that if p is impossible it will entail every possible proposition, which seems unacceptable.
Gist of Idea
Entailment is logical requirement; it may be not(p and not-q), but that has problems
Source
Thomas Mautner (Penguin Dictionary of Philosophy [1996], p.169)
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.169
A Reaction
The word 'entail' was introduced by G.E. Moore in 1920, in preference to 'imply'. It seems clear that we need terms for (say) active implication (q must be true if p is true) and passive implication (p must be false if q is false).