• 国際こくさい — international
• 交際こうさい — association, socializing
• 実際じっさい — reality, actual
• 際限さいげん — limit, bounds
• 水際みずぎわ — water's edge, shoreline
• この際さい — on this occasion

際 is made up of 際 (phonetic + meaning component) and 阜（こざとへん）, which often represents a place or terrain. The meaning of "border" or "edge" comes from the idea of a boundary between spaces or states — whether physical (as in "edge of water") or abstract (as in "on this occasion").

出発しゅっぱつの 際さいには、必かならず 連絡れんらくしてください。
Please be sure to contact us at the time of departure.

彼かれと 会あったのは 旅行りょこうの 際さいでした。
I met him on the occasion of a trip.

The onyomi サイ appears in most compounds like 国際 and 実際. The kunyomi きわ is often used to mean "edge" or "brink" and appears in more descriptive expressions or classical phrasing.