1.
Western-style paper; machine-made paper
Paper manufactured using Western methods, as opposed to traditional Japanese washi. The standard paper used for printing, writing, and everyday purposes.
洋紙 is the technical term for Western-style machine-made paper, as opposed to traditional Japanese 和紙.
Forms
洋 (Western) + 紙 (paper)
Usage
In everyday conversation, people simply say 紙 since Western paper is the default. 洋紙 is mainly used when specifically contrasting with 和紙, or in technical and industrial contexts.
Do not confuse with the homophone 用紙 (paper/form for a specific purpose), which is far more common in daily life.