You give a good description of arbitrage. A more formal definition ( taken from the book "Arbitrage Theory in Continuous Time" by Tomas Björk ),

The portfolio h is an arbitrage portfolio if it satisfies the conditions

V^h_t=0 = 0
P(V^h_t=1 >= 0) = 1
P(V^h_t=1 > 0) > 0

Where V^h_t is the value process at time t for the portfolio h, where h is defined as a vector of different assets.