While your primary keyword usually anchors the title, related terms can add useful context in the meta description and in secondary title elements.
Keep the Primary Keyword Prominent in the Title
The exact-match phrase still deserves the most prominent placement in the title tag, since it's the clearest, most direct relevance signal.
Use the Meta Description for Supporting Context
A related term or two in the description can add helpful specificity without displacing the core promise the description is making.
Avoid Cramming Multiple Terms Into a Short Space
Both title tags and meta descriptions are short enough that overloading them with keywords, related or not, usually reads as unnatural.
Next step: Use the LSI Tool to find related terms worth working into your meta description alongside your primary keyword.