a thin film
A soap bubble's shimmering skin, the gold leaf on a picture frame, the anti-reflection coating on your glasses — all are thin films: layers of material spread over a surface, anywhere from a single atomic layer to about a micron thick (a micron is a thousand nanometres). What makes a thin film its own subject is that it is grown on top of something else, the substrate, so its structure is shaped as much by that surface underneath and by how the film was laid down as by the material's own preferences. A film is not simply a very flat piece of bulk.
Films are built up atom by atom or molecule by molecule — by evaporating material and letting it condense, by knocking atoms off a target with an ion beam (sputtering), or by reacting gases at the surface (chemical vapour deposition). How the first atoms arrange themselves sets the whole structure, and there are three classic growth modes: the arriving atoms may spread out to wet the surface layer by layer (Frank-van der Merwe), clump into separate three-dimensional islands (Volmer-Weber), or start flat then break into islands (Stranski-Krastanov). Once growth thickens, atoms often pile up into tall, parallel columnar grains, and the crystallites frequently share a preferred orientation, a texture, imprinted by the substrate and the growth direction.
This dependence on substrate and process means a thin film can have a structure quite unlike the same material in bulk: different grain shapes, strong texture, built-in stress, even phases that would not be stable on their own. That is a feature, not a bug — it is exactly how microchips, hard tool coatings, mirrors, solar cells, and magnetic disks are engineered, by choosing a substrate and a deposition recipe to coax the structure you want. When the film grows in crystalline registry with the substrate, the special case is called epitaxy.
The reflective coating on a bathroom mirror is a thin film of aluminium, often under 100 nm thick, sputtered onto glass. Under a microscope its structure is not a mirror of bulk aluminium but a dense mat of tiny columnar grains grown perpendicular to the glass, with a texture set during deposition — a structure that exists only because it was grown as a film.
Grown on a substrate, a film develops columnar grains and texture — a structure the bulk material never has.
Thin does not mean thin-as-in-fragile: it means thin relative to its area, so surfaces and interfaces dominate its structure and its behaviour. The substrate is part of the system — change the substrate or the deposition temperature and you can get a different phase, texture, or grain structure from the identical source material.