1. The boundary between Maryland and Pennsylvania; symbolic dividing line between North and South before the American Civil War
   - Mason-Dixon line, Mason and Dixon's line

Type of: state boundary, state line

• order Trogoniformes
• order Tuberales
• order Tubulidentata
• order Tulostomatales
• order Ulvales
• order Umbellales
• order Uredinales
• order Urodella
• order Uropygi
• order Urticales
• order Ustilaginales
• order Volvocales
• order Xiphosura
• order Xyridales
• order Zeomorphi
• order Zygnemales
• order Zygnematales
• order-Chenopodiales
• ordered
• ordered series
• orderer
• ordering
• orderliness
• orderly
• orderly sergeant