Periods are the complex numbers which may be defined by integrals of algebraic functions (with rational coefficients) over domains defined by algebraic inequalities (with rational coefficients). They include all algebraic numbers. The philosophy of motives predicts that classical Galois theory for algebraic numbers is a special case of a Galois theory for periods.
Links:
[1] https://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] https://www.mpim-bonn.mpg.de/node/3444
[3] https://www.mpim-bonn.mpg.de/node/246