Hiraeth is a Welsh word that is somewhat difficult to describe in English, for the reason that there is no single English word that expresses all that it does. Some words often used to try to explain it are homesickness, yearning, and longing.
However, there is more depth to hiraeth than in any of those words on their own. It seems to be a rather multi-layered word, which includes a different variety of homesickness than what is generally referred to. This kind of homesickness is like a combination of the homesickness, longing, nostalgia, and yearning, for a home that you cannot return to, no longer exists, or maybe never was. It can also include grief or sadness for who or what you have lost, losses which make your “home” not the same as the one you remember.