Arbitrage is technically a set of simultaneous transactions where you buy and sell at the same time. Purchasing something and reselling it later for a higher price is not arbitrage.
It is usually used to describe a riskless transaction.
I'm not sure I agree with the description of arbitrage as "riskless". The arbitrageur is accepting the risk that the arbitrage opportunity will go away in between the buy and the sell. This is true whether you're arbitraging salt via caravans across the Sahara or arbitraging currencies in millisecond trades.
It's the idea of simultaneous transactions that makes it riskless. Whether millisecond forex trades can be modeled as "simultaneous" is another question.
If you want to get more precise, the benefit of simultaneous or near simultaneous transactions is reduced risk that prices will fluctuate. Arb is just an exploitation of different prices in different markets.
It is usually used to describe a riskless transaction.