The term AIRLESS is used to describe a space or environment that is lacking in air or oxygen. This can be seen in the crossword solutions given, such as CLOSE (5 letters), which means having little or no space between two things; OPPRESSIVE (10 letters), which means causing a feeling of being weighed down or burdened; MUSTY (5 letters), which means having an unpleasant smell caused by lack of fresh air; SWELTERING (10 letters), which means extremely hot and humid; STUFFY (6 letters), which means having an unpleasant smell caused by lack of fresh air; FROWSTY (7 letters), which means having an unpleasant smell caused by lack of fresh air; and FLAT (4 letters), which means having no air or oxygen. All of these words are used to describe a space or environment that is lacking in air or oxygen, making it airless.