Hold ⌃ Control+⇧ Shift while pointing at any word, or press ⌥ Option+⌘ Command+I with the pointer over it. The Word Information popup appears just above the word without taking focus from the app you’re in.

Word Information popup for the word understand: a symbol, the syllables un-der-stand, Synonyms and Define buttons, and a list of synonyms including explain, interpret and realise.
The Word Information popup for understand, with its synonyms shown.

The popup shows:

  • a symbol for the word, where one is available
  • the word broken into syllables
  • Synonyms — shown when the word has synonyms
  • Define — the definition from the macOS dictionary, shown right in the popup
  • Homophones — shown only when the word has words that sound the same

If there’s no symbol for a word and your Mac is online, the popup may show a photo instead.

The Word Information popup works in any app, including text in images and PDFs.