Can you point your phone camera at a word to get its definition?
Yes. Several iPhone apps read text through the camera and return a definition for a single word rather than translating a whole block. They rely on on-device text recognition, so they respond in about a second and do not need to send a photo anywhere.
The thing that separates a usable one from a frustrating one is what happens to inflected words. The word printed on the page is often not the form the dictionary lists — plurals, past tenses, conjugations. A tool that reads the word and searches for it verbatim will simply fail on much of real prose.
Booktionary reduces the word to its dictionary form before searching, so wended finds wend and mangeaient finds manger. It also keeps the camera open between lookups, which matters more than it sounds: reopening an app for every word is what makes people stop looking words up at all.
Good light helps more than anything else, and holding the phone roughly 15 cm above the page is the sweet spot.