Superwoman is the name given to several fictional characters published over the years by DC Comics, most of them being, like the popular Supergirl, a woman with powers similar to those of DC's highly popular Superman. The name "Superwoman" was originally copyrighted by Detective Comics in an effort to prevent competitors from using it. As was the practice, an ashcan copy was created with the title of Superwoman. The cover was a reproduction of More Fun Comics with the interior content reprinting of the third issue. The first true appearance of Superwoman, which is usually thought to have been one of many trial runs for the future introduction of Kara Zor-El as Supergirl years later, was printed in Action Comics.
The first appearance of the name "Superwoman" in a DC comic was a story in Action Comics where Lois Lane dreams that she has gained superpowers from a blood transfusion from Superman and launches a career as Superwoman.
The theme would be revisited in a 1947 Superman comic in which a pair of fraudulent magicians cast a "spell" on Lane, making her believe she has superpowers. Superman is forced to play along with the ruse for a time, using super-speed to invisibly intervene in Lane's adventures, supporting the illusion. She briefly sports a costume modeled on Superman's before the spell is "broken". A story from Action Comics has Lois actually gaining superpowers thanks to one of Lex Luthor's inventions, and launches a short-lived career as "Superwoman."
Later stories would sporadically feature tales in which Lois gained superpowers and functioned as a "Superwoman" of sorts, but all of these were, like the 1951 tale above, only temporary, with the powers wearing off by the end of the story. A typical example of this is "The Turnabout Powers" from Superman Family, where the Earth-Two Lois Lane gained powers from her husband (the Earth-Two Superman) through the unexpected effect of an exotic extraterrestrial plant Superman had brought into their home. The plant's death reversed the effect. Another example would be in the Batman/Superman: World's Finest mini-series where Mr. Mxyzptlk briefly transformed Lois into a "Superwoman" with costume and powers.
At the end of All-Star Superman #2, Lois Lane is presented with a formula called "Exo-Genes" created by Superman that allows her to have his powers for 24 hours, and she became Superwoman. During her adventures with her new Kryptonian powers, she was wooed by two superhumans named "Samson" and "Atlas", and was captured by a time-Ultrasphinx. Her powers faded away at the end of the day. Notably, her costume seemed to be exactly the same as that of the Anti-Matter Universe's Superwoman, but in Superman's colors. Both outfits were designed by Frank Quitely.