Sentence:  The window - sign of human life, wink to the passerby, eye of the building allowing one to gaze at the outside world without being seen, <WSD>welcomer</WSD> of the daylight and the sun's ray highlighting surfaces and objects, source of fresh air and sometimes place of exchange of words and smells . . . but also a break in the wall's structural continuity, and thus place of vulnerability, fragility, thermal sensitivity, leakage. 
Meaning of the word marked with <WSD> tags:  Something or someone that greets or is present for the arrival of something.