• 欠席けっせき — absence
• 欠点けってん — shortcoming, flaw
• 欠かける — to be lacking
• 欠乏けつぼう — shortage, scarcity
• 欠陥けっかん — defect

The radical 欠 means “yawn” and depicts a person with mouth open and body bent. It visually conveys “lacking” or “opening,” and from there the meanings of “gap,” “fail,” or “missing” emerged. It's a key kanji for expressions involving deficiency or absence.

彼かれは 会議かいぎを 欠席けっせきしました。
He was absent from the meeting.

その製品せいひんには 欠点けってんがありました。
That product had a flaw.

The on-yomi ケツ is commonly used in compound words like 欠席 or 欠点. The kun-yomi かける and かく describe situations where something is missing or incomplete.