Parameter list

Common parameters:

+a         Semimajor radius of the ellipsoid axis
+alpha     ? Used with Oblique Mercator and possibly a few others
+b         Semiminor radius of the ellipsoid axis
+datum     Datum name (see `proj -ld`)
+ellps     Ellipsoid name (see `proj -le`)
+k         Scaling factor (old name)
+k_0       Scaling factor (new name)
+lat_0     Latitude of origin
+lat_1     Latitude of first standard parallel
+lat_2     Latitude of second standard parallel
+lat_ts    Latitude of true scale
+lon_0     Central meridian
+lonc      ? Longitude used with Oblique Mercator and possibly a few others
+nadgrids  Filename of NTv2 grid file to use for datum transforms
+no_defs   Don't use the /usr/share/proj/proj_def.dat defaults file
+pm        Alternate prime meridian (typically a city name)
+proj      Projection name (see `proj -l`)
+south     Denotes southern hemisphere UTM zone
+to_meter  Multiplier to convert map units to 1.0m
+towgs84   3 or 7 term datum transform parameters
+units     meters, US survey feet, etc.
+x_0       False easting
+y_0       False northing
+zone      UTM zone

Extended list provided by Gerald Evenden “grepped out of the RCS directory”.

+a         Semimajor radius of the ellipsoid axis
+alpha     ? Used with Oblique Mercator and possibly a few others
+azi
+b         Semiminor radius of the ellipsoid axis
+belgium
+beta
+czech
+e         Eccentricity of ellipsoid = sqrt(1 - b^2/a^2) = sqrt( f*(2-f) )
+ellps     Ellipsoid name (see `proj -le`)
+es        Eccentricity of the ellipsoid squared
+f         Flattening of the ellipsoid (often presented as an inverse, e.g. 1/298)
+gamma
+geoc
+guam
+h
+k         Scaling factor (old name)
+K
+k_0       Scaling factor (new name)
+lat_0     Latitude of origin
+lat_1     Latitude of first standard parallel
+lat_2     Latitude of second standard parallel
+lat_b
+lat_t
+lat_ts    Latitude of true scale
+lon_0     Central meridian
+lon_1
+lon_2
+lonc      ? Longitude used with Oblique Mercator and possibly a few others
+lsat
+m
+M
+n
+no_cut
+no_off
+no_rot
+ns
+o_alpha
+o_lat_1
+o_lat_2
+o_lat_c
+o_lat_p
+o_lon_1
+o_lon_2
+o_lon_c
+o_lon_p
+o_proj
+over
+p
+path
+proj      Projection name (see `proj -l`)
+q
+R
+R_a
+R_A
+rf        Reciprocal of the ellipsoid flattening term (e.g. 298)
+R_g
+R_h
+R_lat_a
+R_lat_g
+rot
+R_V
+s
+south     Denotes southern hemisphere UTM zone
+sym
+t
+theta
+tilt
+to_meter  Multiplier to convert map units to 1.0m
+units     meters, US survey feet, etc.
+vopt
+W
+westo
+x_0       False easting
+y_0       False northing
+zone      UTM zone