Spotlight dictionary tool works inconsistently

The short version of the answer is to use “define word” instead of “word meaning”.

There is no single dedicated toggle or on/off switch in Apple's settings specifically for dictionary results inside Spotlight. Spotlight automatically ranks results based on how it interprets your query, which is why it can feel inconsistent or hit-or-miss. 

However, you can use a few reliable tricks and settings to make dictionary definitions appear much more consistently:

Force the Dictionary Result with a Keyword

• Add "define" or "def": Typing just a single word can sometimes trigger a web search, contact, or app result instead of the dictionary. Typing define [word] or [word] definition forces Spotlight to prioritize and display the dictionary card immediately. 

Check and Manage Your Active Dictionaries

Apple uses its built-in system dictionary for these lookups. If the dictionary package for your language is missing or disabled, lookups may fail: 

• Open the Dictionary app, go to Dictionary > Settings (or Preferences) in the top menu bar, and ensure the languages/dictionaries you use are enabled.

Alternative Quick-Lookup Methods (No Spotlight Needed)

If Spotlight continues to skip the definition, you can use built-in shortcuts that don’t require opening the full Dictionary app:

• Mac: Move your cursor over any word and press Command + Control + D (or Force Click or three-finger tap on a trackpad, check System Settings > Trackpad > Point & Click tab > Look up & data detectors setting) to pop up an instant definition bubble anywhere on your screen.
• Siri: Ask “Define [word]” to have Siri show or speak the meaning instantly

More info from Apple:

Find what you need with Spotlight on Mac - Apple Support

Dictionary User Guide for Mac - Apple Support