The mother and daughter were finally reunited at the airport after years of separation.

reunited 是动词 reunite 的过去分词形式。reunite 由前缀 re-（表示“再次”）和动词 unite（联合、结合）组成，字面意思是“再次联合”，即“重聚、团聚”。在句中表示"被团聚"的被动含义。