I love LookUp! I love words and dictionaries. I own lots of dictionary apps. I teach English to non-English speakers, ESL. Contrary to another reviewer or two, I find this to be a unique, browsable, and helpful vocabulary builder. It's NOT a simple dictionary, a reference, that's not the best purpose - it's something to sit and spend time with. I wouldn't recommend it solely to use as a dictionary for quick look ups. I don't need such interesting distraction, I would stay in LookUp , get lost in words, and not get back to my work.
Also contrary to another reviewer, they are NOT words common to daily ordinary American English speakers. You may have at least heard many words, studied them for testing, and even gleaned the meanings from context. There will be words you've heard many times and may even use but you never looked them up yourself, those words you can use and describe with examples but cannot cite a definition. So it's very interesting.
You get visual imagery, etymology, grammar and related words. And links to more comprehensive definitions, a wiki. The presentation of each word is colorful and appealing without cartoonish sensibilities. By simply visually browsing you can learn, hone, or refresh your vocabulary without the tedium of drilling, memorizing, or winding thru long repetitious dictionary definitions.