1. A soldier or guard whose job is to keep watch and protect an area
The sentinel stood at the gate all night long.
A sentinela ficou de pé no portão a noite toda.
2. A person or thing that keeps watch or acts as a warning
The lighthouse served as a sentinel for ships at sea.
O farol serviu como sentinela para os navios no mar.
3. In computing, a special value used to mark the end of a data structure
The programmer used a sentinel value to terminate the loop.
O programador usou um valor sentinela para encerrar o laço.