There is probably an app for this somewhere, but since you have the iPad Air 2 you could easily use the Notes app for this.
Swipe your finger from the right edge of the screen and you'll get a list of apps in slide over view. Pick the Notes app, and create a note for you Word List. Now, whenever you encounter a word you want to save, just select and copy it, slide out the Notes app, and paste it into your word list.
If copy and paste does not work, you may have to type in in manually.
That's the most cumbersome way. Once the note has been created you can use the share sheet. Select the word (long press) and choose Share in the pop-up menu, then the notes app icon from the share sheet. The Notes dialog will let you choose creating a new note, or adding the text to an existing one.
Notes:
1: If you use the share/action icon (box with up arrow in the tool bar) then Safari saves a link to your note, not the selected text. Not even if you have copied text into the clipboard. Other apps may have special link/content saving features like this, bypassing the clipboard text.
2: I had trouble using the share option with single words on the Wikipedia site. Don't know why, but it suggests that some sites may not work well this way. When I forced a Desktop version of the Wikipedia site, the method worked again. When the Share option did not work, it failed to pull up the share sheet after tapping Share.
3: To force a Desktop version of a site in iOS 9, Safari, tap hold the reload button in the URL field.
4: If the Share option in the pop-up menu does not appear, you'll have to use the cumbersome method of copy and pasting directly into the note.
Here is a screen shot where I select a word in iBooks. Notice that besides the Share option, you could look up the definition immediately.
After I choose Share, I tap the Notes icon in the share sheet, and get this dialog. Notice the Choose Note dialog at the bottom.
Here is the resulting note in slide over view. The previous entries were added from Safari.
Anyway, it's something you can use and/or play with until you find a better solution.