Splined refers to the incorporation of small ridges inside the tube. It increases the tube's stiffness with minimal weight increase. The concept was created by Columbus for their steering columns. It was then added to the bottom bracket ends of seat tubes in the early 1980s, to add a little more material for front derailleur braze-ons which were tearing off of standard tubes. In 1984 Columbus introduced their SLX and SPX tubesets which had splines at the bottom bracket end of the seat and down tubes, and chainstays, to noticeably stiffen up the bottom bracket. Miyata brought out their STB (spline triple butted) tubeset in 1986 but carried the concept further by adding splines to the top tube, and fork blades.

For pics of splined tubing, see posts #10 and 11 of this thread: http://www.bikeforums.net/showthread.php?t=461865