The phrase "estimate by the eye" refers to the act of guessing or approximating something based solely on visual observation. There are several synonyms for this phrase including "eyeballing," "guesstimating," "approximating," and "ballparking." Another term that is commonly used is "roughly calculating," which implies that the estimate is not exact but is within a reasonable range of accuracy. These terms are often used in contexts where precise measurements are not necessary, such as when estimating the weight of produce at a farmer's market or the distance between two objects in a photograph. Regardless of the term used, the process of estimating by the eye requires a certain level of judgement and experience.