Olam katan (Hebrew עולם קטן "Small World") is a concept of Jewish philosophy that certain concepts mirror (in a kind of "microcosm") the world as a whole (the "macrocosm".) Its use probably originates from the Midrash (a section of the Midrash collection Otzar ha-Midrashim bears the title).
Man is compared to a "small world". This is mirrored in the Talmudic statement that Adam's head originated in the Land of Israel and his trunk in Babylon (Tractate: Sanhedrin). It is similarly echoed in the famous statement that killing one person is comparable to destroying a whole world (Mishnah Sanhedrin 4:5).
In other Midrashic sources, the Tabernacle is referred to as a "small world", with its various objects mirroring both different astronomical concepts and certain organs in the human body.