What is a “regiment”?

“The regiment was the basic maneuver unit of the Civil War.  They were recruited from among the eligible citizenry of one or more nearby counties and usually consisted of 1,000 men when first organized.  The attrition of disease, combat, and desertion would rapidly reduce this number.  Replacements were exceedingly rare for both sides–it was more typical for an entirely new regiment to be raised instead.  Regiments were usually led by colonels.” — Civil War Trust, History: Civil War Army Organization