A typographical error or typo is a mistake made by accident while typing or transcribing something. Most people distinguish between accidental errors like typos and errors of ignorance like poor spelling and grammar. These errors are extremely common, and very frustrating for editors since they can sometimes hide well enough that they are not caught before publication. Some typing errors are so common that they have become running jokes in some communities; “teh” is sometimes used for “the,” for example, since many people transpose these letters so frequently.

One of the most common typing mistakes is a transposition of letters, in which all of the letters needed to spell a word are present, but in the wrong order. This sort of typo often occurs when someone is typing rapidly and not paying enough attention to his or her work. It is also not uncommon to see substitutions of letters caused by a slip of the fingers on a keyboard.

Duplications and omissions are also common print errors; for example, someone might type a word twice in a row without realizing it, leading to a duplication error. In other cases, someone might accidentally leave a word out, or, more commonly, leave a letter out of a word to cause a typo. Duplication errors can be tricky to catch if a written work is only lightly skimmed, and if an omission error spells a word by accident, it can make the typo challenging to find.

Spelling and grammar errors are not considered typos because they are not typographical errors, they are signs of ignorance in which the typist's mind, rather than the printed material, must be corrected. In some cases, someone will type a homophone for a word by mistake, in which case the error may be referred to as a “thinko,” acknowledging that the error is grammatical in nature, but caused by a genuine mistake. Some common thinkos include the substitution of its for it's, or there for their, and vice versa.

Preventing typos and catching them when they occur is very important to most people who produce printed material. This is why scrupulous editing by another person is extremely important, since it can trap basic typos which the author might miss. On a computer, the use of spelling checks can help to catch fat fingered errors, but nothing substitutes for a careful visual review; a typing error can be extremely glaring and distracting to a reader.

It is also extremely important to check for typos in computer coding, since a single typo can render a program useless. This is also true with programming languages like HTML, which can be very unforgiving of typographical mistakes.