Stephen Strasburg swears he's not throwing a slider. We know this, because MLB.com's Jamal Collier specifically asked him about it after Strasburg set the Nationals' franchise record for strikeouts in Wednesday's 3-1 win over the Braves in Atlanta, and the pitcher clearly denied it.
Perhaps Strasburg is not, though we know he's been playing with one on and off for years. Maybe he just doesn't refer to the pitch as a "slider." Could be that new pitching coach Mike Maddux has introduced some tweaks that make an existing pitch act differently. Or maybe, in the interest of not wanting to give away his secrets, Strasburg simply prefers not to discuss it.
Whatever the truth is, there's no denying the fact that in Strasburg's first start of the year, something was different, because the man who almost exclusively was a four-pitch pitcher in 2015 -- two fastballs, a changeup and a curveball -- very clearly had a fifth in Atlanta, and it certainly looked like a slider.
Just look at this graph that compares his Statcast™ spin rate to his pitch velocity from Wednesday. You can pretty clearly see the curveball (orange) being the slowest pitch, in the low 80s. The changeup (blue) hovers in the high 80s; the pair of fastballs (red and yellow) are similarly clustered around 95 mph. But at the bottom, there's clearly a fifth pitch (green), and it certainly looks like a slider:

For reference, that pitch -- whatever you call it -- averaged 89.3 mph and 2,275 rpm in Atlanta on Wednesday. While those are both above average for the pitch, that's not terribly surprising given that Strasburg is above average in a lot of things, especially with how dominant he was after returning from injury last year. Matt Harvey, for example, thew his signature slider at 89.3 mph last year. Santiago Casilla is probably the closest righty comp in terms of velocity and spin, and he allowed just two extra-base hits in 131 sliders thrown in 2015.
Against the Braves, it ended up being a pretty intriguing new tool. Strasburg threw it 12 times, and it so interested the Braves that they ended up swinging at it 11 times, which turned into two strikeouts and five balls in play, two of which were base hits (one an infield hit to David Murphy).
It sure looked useful enough when Strasburg used it to get a strikeout from Bud Norris in the first:
Gif: Stephen Strasburg strikes out Bud Norris
Remember the movement on that, and compare that to what his four-seam fastball looks like when it's coming in high at you, like when A.J. Pierzynski struck out in the sixth:
Gif: Stephen Strasburg strikes out AJ Pierzynski
So is it a slider? Who knows. Call it what you like. Strasburg doesn't seem interested in using the term, but it's hard to ignore the evidence that it sure looks like one. Whatever it is, he threw it a dozen times on Wednesday, after throwing it all of 40 times over the last two years combined. It doesn't even have to be good, though it looks like it could be. It just has to exist. After all, the last thing hitters need is another thing to worry about from Strasburg. He's hardly short on weapons.