Spellcheck is a feature included with various operating systems and applications that checks text for spelling errors. Some spellcheckers must be run manually, while others check each word as you type.
The first spellcheckers were included with word processors, such as Microsoft Word and Corel WordPerfect. They worked by checking each word in a text document against a list of words in the program's "dictionary." If no match was found, the spellchecker would stop and display an alert saying the word was misspelled.
Modern spellcheckers work in a similar way, but include several advancements. For example, most spellcheckers now allow you to customize the dictionary by adding words so they won't be flagged as misspellings in the future. Microsoft Word's spellchecker checks grammar as well as spelling. For instance, it may tell you to use a singular pronoun rather than a plural one or suggest that you use "which" instead of "that" after a comma in a sentence.
Most spellcheckers now perform spellchecks in real-time, underlining misspelled words (typically in red) as you type. OS X will often correct common spelling mistakes automatically (for example, changing "teh" to "the"). You can also right-click a misspelled word in OS X and select the correct spelling from a list of options. If the word is spelled correctly, you can choose "Learn Spelling" to add the word to the OS X dictionary.
Mobile devices, such as smartphones and tablets take spellchecking to a whole new level with autocorrect and autocomplete. For example, iOS and Android automatically correct misspellings by guessing what letters you meant to press on the keyboard. Microsoft Windows Phone provides spelling suggestions as you type, which allows you to complete words by simply selecting them from a list.
NOTE: Spellcheck may be used as both a noun (performing a spellcheck) and a verb (spellchecking a document). Ironically, there is no official spelling of the term, as "spell-check" and "spell check" are also acceptable. However, "spellcheck" is most common.