drop-in
adjective [ before noun ] uk/ˈdrɒpˌɪn/ us/ˈdrɑːpˌɪn/
（咨询、服务中心等）不用预约的
used to describe a place where people can go, usually to get help or advice, at any time without making an appointment (= an agreed date and time)
a drop-in centre for the homeless 为无家可归人士提供的开放中心
a drop-in clinic/shelter 无须预约诊所/开放式庇护所