Transient execution is CPU behavior where a processor starts executing instructions before it knows whether they should really be allowed to complete. This happens during speculation, such as when the CPU guesses the outcome of a branch or the location of data. If the guess is wrong, the architectural results are discarded, but the work may still leave traces in caches, branch predictors, or timing behavior.
Security teams care because those traces can be measured and turned into side channels. In attacks like Spectre, an attacker uses transient execution to make a processor touch secret-dependent data, then infers the secret from the residue left behind. The issue is not a normal memory bug; it is a leak created by performance features. Defenses include microcode updates, speculation barriers, compiler mitigations, kernel hardening, and reducing exposure in high-risk code paths such as browsers, JIT engines, and shared cloud environments.