For actual authoritative word definitions, I use Oxford English Dictionary - it requires an account but most libraries include free access.
For quickly finding synonyms and other uses while writing, my go-to for the last few years has been WordHippo. There are plenty of other sites that give synonym lists, but in my experience WordHippo surfaces way more variety, including idiomatic expressions and slang.
For actual authoritative word definitions, I use Oxford English Dictionary - it requires an account but most libraries include free access.
For quickly finding synonyms and other uses while writing, my go-to for the last few years has been WordHippo. There are plenty of other sites that give synonym lists, but in my experience WordHippo surfaces way more variety, including idiomatic expressions and slang.