also: ほそせん
noun
thin line; fine line
Refers to a physically thin line, such as in drawing, design, or technical contexts.
この図面では、細線が補助線を表している。
In this drawing, the thin lines represent auxiliary lines.
細線で輪郭を描いてください。
Please draw the outline with a fine line.
太線 means 'thick line', the opposite of 細線.
Compound of 細 (さい, 'thin') and 線 (せん, 'line'). The reading ほそせん uses the kun'yomi of 細.