A spell-check is the tool inside a computer program — Word, Google Docs, your phone's keyboard, a browser — that looks at what you have typed and marks the words it does not recognise, usually with a red wavy underline. You can then right-click and pick the spelling it suggests. The word is used two ways. As a noun it can mean the tool itself ("the spell-check didn't catch that") or the act of checking ("give it a spell-check before you send it" — meaning run the check over the whole document once). As a verb it means to run that check: "I spell-checked the report", "have you spell-checked it?" Written forms wander: spell-check, spellcheck, spell check all appear, and no one will mark you wrong for any of them. Here is the honest part your teacher should tell you. A spell-check only knows whether a string of letters is a real English word — it does not know whether it is the word you meant. Type "their" when you meant "there", or "form" when you meant "from", and the tool stays silent, because both are perfectly good words. So a clean spell-check does not mean a clean piece of writing, and students who trust it completely hand in essays full of real words in wrong places. Newer tools that check grammar and meaning are better at this, but the plain spell-check still cannot read your mind. It also flags names, place names and technical terms it has never met, which is normal and not an error on your part.