solid line
Refers to an unbroken line in diagrams, maps, or design, as opposed to a dashed or dotted line.
The solid line on the map indicates a national highway.
実線 is a solid, unbroken line, the direct opposite of 点線, a line made of dots.
Compound of 実 (じつ, 'real, solid') and 線 (せん, 'line'). The reading じっせん results from a common gemination (促音便) of 実 before an unvoiced consonant.