ḱom
“beside, with, together”with/together
PIE preposition meaning beside, with, or together. Source of Latin com/con-, English "common," and words for togetherness.
Discussion
*ḱom is a Proto-Indo-European preposition meaning "beside," "with," or "together," expressing accompaniment and collectivity.
Latin cum "with" (older com-) is the primary reflex, yielding the enormously productive English prefix con-/com-/co- (with assimilation: col-, cor-). Examples include common, community, company, combine, connect, congress, conspire, and hundreds more. The prefix remains fully productive in modern English (co-author, co-exist, co-operate).
The palatovelar *ḱ gives Latin c regularly in the centum languages. Old Irish com- "with, together" and Welsh cyf-/cyn- continue the Celtic forms. Germanic does not directly preserve this root as a preposition (replaced by *miþ "with" > English with), but borrowed forms from Latin are pervasive.
Lithuanian sam- and Old Church Slavonic sŭ- "with" (Russian so-/s-) reflect the satem development (*ḱ > s). Sanskrit sám "together" (as in saṃskṛta- "Sanskrit," literally "put together, perfected") preserves the root transparently.
Modern descendants include English common, community, company, combine, connect, congress, conspire, co-, and through Sanskrit: Sanskrit itself (saṃskṛta-).
Notes
Source of Latin "cum/con-" (with), English "co-". Comitative/collective marker.