From Online Dictionary of Crystallography

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The oriented attachment and adherence of individual crystals to form a heterogeneous crystalline edifice is termed synneusis, from the Greek syn (together) and neusis (swimming). Depending on the nature of the individuals and their relative orientation, the result can be a parallel growth, a twin, an allotwin, a plesiotwin or an epitaxy.

The term was introduced in petrology to describe the oriented attachment and adherence of minerals segregated from a magma at an early stage.