合成 (héchéng) is a verb meaning 'to synthesize,' 'to compose,' or 'to combine into a whole.' It is used in scientific contexts for chemical synthesis as well as in general usage for combining elements, such as compositing images or producing compounds.
这张照片是用两张图片合成的。
Zhè zhāng zhàopiàn shì yòng liǎng zhāng túpiàn héchéng de.
This photo was composited from two images.
科学家成功合成了一种新材料。
Kēxuéjiā chénggōng héchéng le yī zhǒng xīn cáiliào.
Scientists successfully synthesized a new material.
这首歌是用电脑合成的声音。
Zhè shǒu gē shì yòng diànnǎo héchéng de shēngyīn.
This song uses computer-synthesized sound.