Proposed by Gilbert N. Lewis, this modern theory focuses entirely on the movement of electron pairs. Older traditional theories only define bases by their ability to accept hydrogen protons or release hydroxide. For a molecule or ion to act as a Lewis base, it needs a very specific feature.
It absolutely must have at least one lone pair of electrons readily available for donation. When a Lewis base donates this pair to an electron-hungry species, they instantly link together. The electron-hungry partner receiving the pair is always known as a Lewis acid.
Together, the newly combined chemical product is called a Lewis acid-base adduct. This concept is incredibly powerful because it explains many complex reactions in coordination chemistry. It perfectly describes how various ligands bind to transition metal ions to form colorful complexes.
The older Brønsted-Lowry theory completely fails to explain these specific metal interactions because no protons actually move. A common student misconception is thinking a molecule can only follow one specific base theory. Many common chemicals act as both Brønsted-Lowry bases and Lewis bases depending on the exact reaction. However, the Lewis definition is much broader and covers a much wider variety of chemical reactions.