The first thing to try is Command F right where you are. A search box appears and highlights every match on the page, one at a time. If that finds nothing, the word is pixels instead of text, which is a different problem and needs OCR. If you need to find a word across files, Spotlight and Finder search the disk instead.
Press ⌘F in Safari, Preview, Pages, Notes, Word, or any app. A search box appears usually in the upper right corner. Type the word you are looking for.
Every match is highlighted as you type. Arrow buttons let you jump to the next one or the previous one. This works the same in every app because it is built into macOS text views.
Command F finds nothing. You drag across it and nothing selects. This means the word is not actually text, it is part of a picture. Scanned pages, screenshots, video frames, images. All the same.
Preview can help if the picture is still. Open it in Preview and look for Live Text in the Tools menu. It recognises words in still images and lets you select and copy them once they are recognised.
For everything else, OCR tools read the pixels. Leafy has OCR built in. Press ⌥A and box the word. Leafy reads the text off that patch of screen and shows you what it found, with a meaning or a translation next to it.
Press ⌘Space to open Spotlight and type the word. Spotlight indexes the text inside documents, mail and notes as well as file names, so a word buried on page 40 of a PDF still brings that PDF up. Put the word in quotes to demand an exact match. What Spotlight does not tell you is where in the file the word is, so open the result and press ⌘F again.
Open Finder and press ⌘F. This is different from the app search. Type your word in the search box, then look at the toolbar above the results. You can click the kind of file you want to narrow down to, or change the scope to search the whole disk instead of just the current folder.
As you type, a small menu offers Name matches. Ignore it and press Return: a plain search looks inside file contents, the Name matches option only looks at file names. The Kind and Date buttons above the results narrow the list, and the This Mac button widens the search from the current folder to the whole disk.
These are two different things. Finding is about locating where a word appears. Looking up is about learning what it means. A browser find button gets you one. A dictionary gets you the other. Leafy does the second one.
If you have the word and need to understand it, go to looking up a word on Mac. If the word is on your screen but you cannot select it, try translating what is on screen.
Free while LeafyApp is in beta. One download, macOS 14 and up.
Also worth a look: looking up a word on Mac, translating anything on screen, or OCR for a PDF.