Türkçe'de 'come back'in karşılığı geri gel(mek)tir.

"come back" Türkçe olarak "geri gel"dir

did nasıl nedir come back ?

If you are longing for someone and want him/her back, you would say "geri dön (dönmek=~return)". It's very rare that Turkish people would use "geri gelmek" in imperative. They would rather say "buraya gel/gel buraya" to mean "come over". "Geri gelmek" rather denotes "come over" than "come back". While literal translation for "come back" is "geri gel", a true translation should be "geri dön" in my opinion.

Geri dönmek. Geri gelmek.