Word for the web checks spelling, and you can add words to your dictionary. However, its built-in word list doesn't show definitions, and you can't lookup words, the way you can with Word for the desktop.
Both Word for the web and Word for the desktop mark misspelled words with a wavy red underline.
To correct a misspelled word, switch to Editing View by selecting EditDocument > Edit in Browser.
Right-click the word, or select the word and select Review > Spelling.
A short menu opens to show alternative spellings and words.
Select the spelling or word you want to use.
For more about checking spelling in Word for the web, see Check spelling in Word for the web.
Note: Adding a word to a custom dictionary in one Office program makes that word available for spelling checks in other Office programs.
If you want to install a dictionary or see definitions and you have Word for the desktop, click Open in Word and follow the steps in Check spelling and grammar. To add words to a dictionary, see Add words to your spell check dictionary.
If you don’t have Word, you can try it or buy it with the latest version of Office.