Forestallment is a somewhat uncommon word that means the act of preventing something from happening, typically in advance. Synonyms for forestallment include prevention, preemption, obstruction, hindrance, interruption, intervention, and deterrence. Each of these words expresses the idea of stopping or hindering something before it occurs, often in order to avoid negative consequences. Other possible synonyms might include anticipation, preparation, or readiness, which suggest a more proactive approach to avoiding problems before they arise. Ultimately, the choice of synonym will depend on the context in which the word is used, and the specific connotations and implications that the writer wishes to convey.