The name first appears on the cover of Flash #93 (August 1994), with an out-of-control Bart Allen fighting the Flash. The cover is captioned, “Brash Impulse!” Over the next few issues, Wally West’s inner monologue refers to Bart as being impulsive, or (at one point) as “Mr. Impulse.”

It first appears on-panel as a name in Zero Hour #3 (September 1994), when Bart meets Superman for the first time, but Bart introduces himself as Impulse. Dan Jurgens writes.

Superman arrives in Keystone to see Velociraptors chasing Bart.

Superman: Dinosaurs from the merging timelines! Need a hand, kid?
Bart: Call me Kid Flash — and get your big “S” handed to you! The name’s Impulse!

After the events of Zero Hour, the name is explained in Flash v.2 #95 (November 1994), written by Mark Waid.

Wally and Bart are running through the city.

Wally: If we’re going to do this hero thing, let’s do it right. You need a code name.
Bart: How about “Kid Flash?”
Wally: Huh, well…
Bart: Pfffft! Just kidding. “Kid Flash.” Yeah, right…. Save yourself the brainpower. You already gave me a name. Repeatedly.
Bart stops and kisses a random woman on the street (yes, Bart).
Bart: I’m Impulse!
Wally: How terrifyingly accurate. And the mask?
Bart: Are you kiddin’ me? I saw so many guys wearing ’em, I just figured it was a law.

Fast forward several years.

Edit: (Thanks to Crazy Penguin for pointing this one out and scanning it) Secret Origins 80-Page Giant (December 1998) tells the origins of the members of Young Justice. In the Mark Waid–written Impulse story, the following exchange appears:

Carol: So who named you Impulse?
Bart: Batman. I have no idea why. Was he being funny?

Eight months later, Todd Dezago had just taken over as the third regular writer on Impulse. He started out with an unusual team-up in Impulse #50 (July 1999): Impulse and Batman against the Joker.

At the end of the issue, Impulse asks Batman a question:

Impulse: Wally said that you named me “Impulse”…what’s up with that?!
Batman: Wally misunderstood. I never meant for Impulse to be your name…. It was meant to be a warning.

Oddly enough, I don’t recall much complaining about the retcon.

Jump forward another half-decade, to the beginning of Bart’s brief career as the Flash. Danny Bilson and Paul Demeo have just taken over as series writers. A dream/origin sequence in Flash: The Fastest Man Alive #2 (September 2006) states, “It was Max who first dubbed me ‘Impulse.’” Fans across the Internet raged at this retcon, even asking about it at conventions, complaining about the “poor research.”

And today, DC released a 6-page preview to Newsarama from the upcoming All-Flash #1 (September 2007). Mark Waid is back as writer. In it, Wally West thinks back on Bart’s life:

“The boy was flaky, unpredictable, and had the attention span of a bullet. Superman code-named him Impulse, and it was probably the first thing that ever stuck to him.”

The funny thing is, all that people seem to remember remember about the naming of Impulse was Batman. Not the name he chose himself.

Update: Part of me (the part that spent two years as an English Literature major) is thinking I should have titled this, “Today we Have Namings of Bart,” but I’m not sure how many people would get the reference.

Update 2: Countdown #43 reiterates the Batman version as Robin speaks at Bart’s funeral: “I remember it was Batman who first called him Impulse… and for a while, the name stuck. I never thought he’d take things seriously enough to be the Flash.”

Update 3 (July 18): All-Flash #1 came out today, and the wording has been altered from what was in the preview. It now simply says, “He was code-named Impulse,” rather than “Superman code-named him Impulse,” side-stepping the question entirely. At this point we have 2 votes for Batman, 1 for Bart himself, and 1 for Max.

Update 4 (September 6): Crazy Penguin at the Flash Message Boards pointed out that there is an earlier reference to Batman naming Impulse: Secret Origins 80-Page Giant (December 1998). That’s 8 months prior to Impulse #50. I’ve updated the article above. We now have 3 references to Batman, 1 to Bart, 1 to Max. (Scan added September 9.)