expansion
不可数名词.意为扩大,扩展.如医院学校等的扩展.eg"The rapid expansion of the university has caused a lot of problems.a这所大学急速扩展,引起了不少问题.
extension
可数名词.
(1)(建筑物的)具体所扩建的部分eg"They are just opened the hospital extension.医院的扩建部分刚刚启用.
(2)额外期限,如将做某事的期限延长,即额外期限.eg"I have applied for an extension to my work permit.我已申请把工作许可证的期限延长.
(3)电话分机.eg"what is your extension number?你的分机号码事多少?