This is a pocket version of the best-selling dictionary in the USA, featuring over 225,000 defined words. Since it is a self-contained application and not a web front-end to an online dictionary, it can be used in situations where your device is in airplane mode.
Merriam-Webster's Collegiate Dictionary isn't cheap -- it sells for US$24.99 in the App Store (click opens iTunes). However, carrying this app on your iPhone is a lot lighter than lugging around the US$23.95, 3.6 lb. (1.63 kg.) printed version.
The other major competition to this application, Webster's New World Dictionary (click opens iTunes) from AcroDesign, is a bit less expensive at US$14.99, but is less complete with only 163,000 entries.
What's the favorite reference you'd like to have on your handheld device? Leave us a comment!