Making words bold

As many have said, of course it will be possible to change all instances of any words/phrases, but InDesign looks for some kind of pattern - if you DON'T want it to apply this bold character style in some cases, there needs to be something that differentiates them from the others. Usually a paragraph style, different size ... etc. When the pattern is broken and consistent, you can control everything.

Do you mean, you need to make the words bold ONLY when used as a definition but NOT when the words are used elsewhere? In that case, you will need to apply a Character style manually whenever you need it. InDesign is not smart enough to know when the same word is used as a definition or when it is normal text--unless there is a pattern to when the definition is used. Then, a GREP F/C or style might be able to work.

If you want the first use of the word in a story to be bold, I suspect a script could do that from a master list.

Hello, i think Grep will be your friend to save time and made what you want 😉
Have a look at the Treasure of Grep group on FB : The Treasures of GREP

I did not get your question entirely, if you apply the bold to your word what exactly happens that your don't want? Anyhow, you could apply a character style to these words, you could also use a grep style to apply this automatically to your document. See one such discussion below

https://community.adobe.com/t5/indesign-discussions/grep-style-bold-specific-words-with/td-p/9943126

If this does not help then please share a sample document explaining what you want in the result.

-Manan