These are my rationalisations for the various par() graphical parameter names. These are not necessarily definitive (some of them are quite a stretch!), but hopefully they are still useful.


adj Text ADJustment (i.e., justification).
ann Draw plot ANNotations? (i.e., labels and titles).
ask ASK before starting a new page?
bg BackGround colour.
bty Box TYpe (what style of box for box() to draw).
cex Character EXpansion factor (text size multiplier).
cex for AXIS tick labels, axis LABels, MAIN plot label (title), and plot SUB-title.
cin Character size in INches.
col Drawing COLour.
Drawing colour for AXIS tick labels, axis LABels, MAIN plot label (title), and plot SUB-title.
cra Character size in RAasters (pixels).
crt Character RoTation angle.
csi Character SIze (height) in inches.
cxy Character size in XY coordinates (relative to the axis scales).
din Device size in INches.
family Text font FAMILY.
fg ForeGround colour.
fig Size of the FIGure region (as a proportion of the inner region).
fin Size of the Figure region in INches.
font Text FONT face.
Text FONT face for AXIS tick labels, axis LABels, MAIN plot label (title), and plot SUB-title.
lab Placement of axis LABels.
las Axis LAbel Style
lend Line ENDing style.
lheight Line HEIGHT (multiplier).
ljoin Line JOIN style.
lmitre Line MITRE limit (when does a pointy line join get beveled).
lty Line TYpe (solid, dashed, ...).
lwd Line WidDth
mai Figure MArgins in Inches.
mar Figure MARgins in lines of text.
mex Figure Margin character EXpansion (multiplier).
mfcol Number of Multiple Figures (use COLumn-wise).
mfg Specify next figure in Multiple FiGure arrangement.
mfrow Number of Multiple Figures (use ROW-wise).
mgp MarGin Position for plot title and axis labels.
new Is the plot region NEW? (not been drawn on).
oma Size of the Outer MArgins in lines of text.
omd Size of the Outer Margins as a proportion of the Device (page).
omi Size of the Outer Margins in Inches.
pch Plotting CHaracter (symbol).
pin Size of the Plot region in INches.
plt Size of the PLoT region as a proportion of the figure region.
ps Text Point Size.
pty Plot TYpe (points, lines, both).
srt String (text) RoTation angle.
tck Length of axis TiCK marks as a proportion of the plot region.
tcl Length of axis TiC marks as a proportion of the height of a Line of text.
usr The axis scale ranges (USeR coordinates).
xaxp X-AXis tick mark Positions.
xaxs X-AXis Style.
xaxt X-AXis Type.
xpd I have NO IDEA where this came from.
UPDATE: Bill Venables has proposed "Do you want to eXceed any of the Plotting Domains?"
yaxp Y-AXis tick mark Positions.
yaxs Y-AXis Style.
yaxt Y-AXis Type.