I had an unexpected encounter with an old classmate in the park.

encounter 是名词，由前缀 en-（表示"进入某种状态"）和 counter（"相反、对抗"）组成，原意是"遭遇、对抗"，引申为"偶然遇见、邂逅"。在这里指"不期而遇的见面"。