Apple does not allow dictionary apps on the App Store which actually use the word definitions built into iOS—they are required to provide their own definitions, which either take up precious storage space or are not available offline.
So, I found some old WTFPL-licensed code on Github, spent an hour or so futzing around to make it compile and look pretty on iOS 12 (because I had no clue what I was doing), and came up with this: https://github.com/Wowfunhappy/Dictionary. It works super well, and I use it every day on my phone.
The only reason I can use this app is because I'm Jailbroken.
• Install a Userscript to de-AMP pages in Google search results.
• Prevent Apple News from saving a history of what articles I read, thus disabling their recommendation engine and preventing a filter bubble.
• Add an extra row of app icons to my homescreen, so I can fit all my apps on one page.
• Get a warning when I set an alarm for PM rather than AM.
...and countless other little things.
Separately, I consider $100 an awful lot of money, especially for a subscription, which I try really hard to keep out of my life.