The 1957 Encyclopédie Larousse defines a cell as follows:
"a small rhythmic and melodic design that can be isolated, or can make up one part of a thematic context. A cell can be developed independent of its context, as a melodic fragment. It can be the source for the whole structure of the work; in that case it is called a generative cell."
The 1958 Encyclopédie Fasquelle defines a cell as follows:
"a term in musical composition, used to discuss cyclic works. It is the smallest indivisible unit; the cell is distinct from the motif, which can be divided; the cell can, itself, be used as a developmental motif."
Source
Nattiez, Jean-Jacques (1990). Music and Discourse: Toward a Semiology of Music (Musicologie générale et sémiologue, 1987). Translated by Carolyn Abbate (1990). ISBN 0691027145.
It is the smallest indivisible unit; the cell is distinct from the motif, which can be divided; the cell can, itself, be used as a developmental motif."