What Does the Word Seeing Mean to an Astronomer Using

What does the word seeing mean to an astronomer using a telescope?

A) the twinkling and blurring of the image due to turbulence in Earth's atmosphere
B) the combined effects of aberrations in the components of the telescope, particularly spherical and chromatic aberration
C) the effect of wind on the telescope that causes it to vibrate, thereby blurring the image
D) the haze or thin cloud in the atmosphere that reduces the brightness and sharpness of the telescope image