Tool

No. You consult a dictionary; you don't consult a word.

Right. I thought not.

Then would this work?

A dictionary is a tool to consult when you want to check an unknown word quickly.
A dictionary is a tool to consult when you want to know the meaning of an unknown word quickly.

Or this?

A dictionary is a tool to look up a word.

None of the three new sentences really work. The word "tool" is not normally used to refer to a dictionary. A dictionary is such a fundamental and important part of the English language that it is usually referred to by its own name, or by a word such as "resource":

We use the dictionary to look up words we don't know.

Develop the dictionary habit.

The dictionary is th

TakaRight. I thought not.Then would this work?A dictionary is a tool to consult when you want to check an unknown word quickly.A dictionary is a tool to consult when you want to know the meaning of an unknown word quickly.

TakaOr this?A dictionary is a tool to look up a word.

These fix the most obvious problem, but I am not

Yes. I don't know why, but to me "a tool+to-infinitive" doesn't really sound right either.

What about "a tool of finding...", GPY? Would it work as fine as "a tool for finding..."?

TakaWhat about "a tool of finding...", GPY?

No, that isn't right.

A dictionary is a good resource to consult when you want to know the meaning of a word quickly.

I agree that "resource" is a better word, but that doesn't necessarily mean that "tool" doesn't work at all, or does it, AS?

Taka that doesn't necessarily mean that "tool" doesn't work at all,

No, it doesn't. However, I don't know why you would want to when you have been told that it is not the best word.