Diffuse nebulae do not have clearly defined boundaries.
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弥漫星云 is the astronomy term — a cloud of gas and dust without sharp edges, contrasted with 行星状星云 (xíngxīngzhuàng xīngyún, planetary nebula) which has a defined shell. 弥漫 itself means 'pervade / fill the air', which captures the formless quality.
