means To respond to something that happens — or, in tech, the JavaScript library that builds user interfaces by re-rendering components when their data changes.

from From the Latin 're-' ('back, again') plus 'agere' ('to do, drive, act') — literally 'to act back.' English borrowed it through the language of 17th-century natural philosophy, where chemicals and forces were said to 'react' upon one another, long before feelings or front-end frameworks claimed the word. Facebook's library kept the older spirit: it acts back when your data shifts.