The term Ronin was originally used to refer to Samurai who had lost their Shogun or Emperor; thusly they had no affiliation or class system to belong to. Most Ronin either had the option of commiting Seppuku (to avoid the shame of no longer having any contributing purpose) or living as a rogue vagaband and mercanaries on the roads of Japan.
In modern times the term has evolved to refer to Japanese students who attempted but failed to pass their university entrance exams; thusly they had no school affiliation for the following year. As most sought-after jobs require a university degree of some type, this usually meant that the "Ronin" student would resort to small, part-time work or remain unemployed for the duration of the year.