Why is the ninth month called September?
mid-14c., "edge of a sea or lake;" late 14c., "space between a block of text and the edge of a page," from Latin marginem (nominative margo) "edge, brink, border, margin," from PIE *merg- "edge, border, boundary" (see mark (n.1)). General sense of "boundary space; rim or edge of anything" is from late 14c. Meaning "comfort allowance, cushion" is from 1851; margin of safety first recorded 1888. Stock market sense of "sum deposited with a broker to cover risk of loss" is from 1848. Related: Margins.
c.1600, "to furnish with marginal notes," from margin (n.). From 1715 as "to furnish with a margin."
margin mar·gin (mär'jĭn)
A border or edge, as of an organ.
A limit in a condition or process, beyond or below which something is no longer possible or acceptable.
An amount that is allowed but that is beyond what is needed.
A measure, quantity, or degree of difference.