Only one definition of the verb "to be" is similar to "to define".

The thing is, "IS" means so many different things that to define it as "to define" would be absurd. The act of definition is that of determining the boundaries of a concept in order to determine its qualities. Being does not, however, presuppose meaning, and the act of being can easily be indefinable. As well, one can attribute meaning to indefinability. As such, it can readily be seen that the two concepts are distinct -- which is one reason for the two different words.