What is the best offline dictionary app for iPhone?
The first thing to check is the download size. Offline dictionaries are large — a serious English dictionary runs to well over a hundred megabytes — so any app claiming full offline coverage in twenty or thirty megabytes is downloading data later or calling a server.
The second is whether the offline dictionary is the whole app or a paid extra. Several apps are free to install and gate offline use, or additional languages, behind a subscription. That is a reasonable model, but it is worth knowing before you rely on it on a plane.
The third is inflection handling, if you are using it while reading. A dictionary that cannot get from wended to wend will disappoint you on the first page of a novel.
Booktionary is built around all three: 345 MB of English and French dictionaries on the device, no subscription and no in-app purchases, $1.99 once, and inflected forms reduced before searching. It is a camera dictionary rather than a typing one, which suits reading and does not suit writing.