The edge, brink, or verge of something; the side or boundary. Used both physically and figuratively (often read as 'kiwa').
崖の際には近づかないでください。
Please don't go near the edge of the cliff.
窓際（まどぎわ）の席に座った。
I sat in the window seat.
道路際の植え込みに花が咲いている。
Flowers are blooming in the planting strip along the road.
A time or moment when something happens; often read 'sai' in formal writing. When used as a suffix it is often pronounced 'giwa' (e.g., 出発の際, 試合のぎわ).
出発の際は忘れ物がないか確認してください。
Please check you haven't forgotten anything before departure.
ご入学の際には必要書類を持参してください。
Please bring the required documents upon admission.
試合のぎわ、選手たちは互いに健闘を称え合った。
At the brink of the match, the players praised each other's efforts.