What Does the Word "Seeing" Mean to an Astronomer Using

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

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