One and two thirds is a way to express a number that is greater than one but less than two. It can be written as the mixed number 1⅔ or as an improper fraction, which is 5/3. This means that there is one whole unit and an additional two thirds of another unit. In practical terms, if you have a pizza cut into three equal slices, one and two thirds means you have one whole pizza plus two out of three slices from another pizza. This concept is useful in everyday situations, such as cooking or measuring ingredients.