  The trending term "filament" is extensively used in the interstellar medium
(ISM) and the star formation community, and is believed to be one of the most
important objects that gauge molecular cloud and star formation. However, the
physical definition of these ubiquitous, elongated, high contrast features is
poorly defined and still actively debated. Despite the absence of a unified
consensus, filaments are believed to be involved in many important physical
processes from galaxy structure formation to the emergence of protostellar
objects. Therefore, understanding how filaments form, what constrains their
growth, and their general physical properties, are extremely important for
theorists and observers who study the dynamics of the ISM and consequent star
formations. This review serves as a collection of the community's views and
develops the concept of "filaments" in the context of the ISM and star-forming
clouds. Observationally, filaments are seen across the entire sky and often
carry an aspect ratio of the order of hundreds. In the context of the ISM,
filaments are believed to form by stretching and tearing from magnetized ISM
turbulence. ISM filaments are subjected to heating and cooling phases, and are
likely to be magnetically aligned. Cold clouds are formed inside ISM due to
turbulence instability. This review updates the understanding of ISM filaments
in the community.
