The weight of a font specifies its boldness. Arranged from lightest to heaviest (more or less):
     hairline, extra light, light
     book, regular, medium
     demibold, semibold, bold, extra bold, heavy
     black, ultra, poster
     
   Here is the table, from the file weight.map.  It is organized
alphabetically by abbreviation.  Each line consists of an abbreviation
and any parts of a PostScript FontName which use that
abbreviation.
     a Thin Hairline
     b Bold
     c Black
     d Demi
     h Heavy Heavyface
     j ExtraLight
     k Book
     l Light
     m Medium
     p Poster
     r Regular Roman
     s Semibold
     u Ultra UltraBlack
     x ExtraBold ExtraBlack