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A grain in a polycrystal is a region whose crystal structure has approximately one common orientation, separated from differently oriented regions by grain boundaries. The atoms remain ordered within each grain even when a macroscopic specimen contains a complicated mosaic of orientations. A grain is therefore different from an amorphous patch or a separate chemical phase.

The boundaries enforce geometric compatibility between neighboring crystals and can obstruct dislocation glide, provide rapid transport paths or act as sinks for defects. A mechanical measurement over many grains averages both the intragrain response and those boundary effects. Observing a prepared cross-section can reveal grain shape, but a single two-dimensional section need not directly give every grain's three-dimensional size. This distinction matters when comparing an elongated grain structure with one described by a single mean grain size.