Remarkable, Marked
い adjective
The kanji means author, but if you think about it, what do authors do? They write about remarkable or marked things, whether it's fiction or nonfiction.
いちじるしい
Something is so remarkable you let out one jeer (いちじる) at the opposing team. Just one, because you're not that mean.
親馬鹿かもしれませんが、娘の英語力の上達が著しい気がするんですよね。
I know I am a doting parent, but I think there has been remarkable improvement in my daughter’s English skills.