DFA
Also known as: Deterministic Finite Automaton, Deterministic Finite Accepter
A DFA (deterministic finite automaton / accepter) is a finite-state machine that, from every state and every alphabet symbol, has exactly one next state. No guesswork, no ε-moves.
Formally $M = (Q, \Sigma, \delta, q_0, F)$ with total $\delta: Q \times \Sigma \to Q$. Run a string left to right from $q_0$; accept iff you land in an accepting state in $F$. That is the same job as a hard-coded validator or a compiled regex matcher after subset construction.
DFAs and NFAs describe exactly the regular languages. The DFA may need exponentially many states versus an equivalent NFA, but membership is then a single deterministic walk: one state, one symbol, one step.