The ability of AI models to think step-by-step, decompose complex problems, and arrive at logically sound conclusions. Modern reasoning models (like OpenAI's o1/o3 and DeepSeek-R1) are trained to generate explicit reasoning traces before answering, dramatically improving performance on math, coding, and logic tasks. This is distinct from simple pattern matching — reasoning models can solve problems they've never seen before.
Why it matters
Reasoning is the frontier capability that separates "AI that sounds smart" from "AI that is smart." Models that reason well can debug code, prove theorems, plan multi-step strategies, and catch their own mistakes. The gap between models with and without strong reasoning is the biggest quality differentiator in AI right now.