"Sight words" is the term for words that readers should recognize instantly. Recognition is important because sightwords are so frequently used—making up, by some estimates, 50 to 75 percent of all words that children are likely to encounter. It is also important because many of them do not sound as they are spelled, making them difficult to sound out using knowledge of phonics. Having the sightwords within his or her repertoire gives the child a better chance to grapple with more difficult and infrequent words without losing the sense of what is being read.
The sightwords include commonly used words—words that are ubiquitous and important to understanding.