William Shakespeare's Sonnet 116 provides a definition of love. Curiously though, Shakespeare does not tell readers what love is by defining what it is; instead, Shakespeare defines love by defining what it is not.
The language he uses is one which speaks to the defining of something through the things which cannot align with it. (Sounds different, but (sometimes) one's knowledge of what something is not allows them to identify what something is). For example, milk is not a fruit because it does not grow on trees, it does not have seeds, and it does not qualify as a solid.
Therefore, according to the sonnet, love is defined by the things which it is not: love is not something which can be altered, bent, or removed. Love is not wandering, shaken, or "Time's fool." By identifying what love is not, one can identify what love is: love is unalterable, unbending, and steadfast. Love is rooted, strong, and "unfoolish."