Virtual Machine
The standardized computer defined by a blockchain's rules, which every node simulates so contracts behave identically everywhere.
The machine takes the current state and a transaction and produces a new state, and it must be completely deterministic: no randomness, no reading the clock, no network access, because thousands of nodes have to reach the same answer independently. That determinism is exactly why oracles exist, since a contract cannot fetch anything by itself. Different networks define different machines, including Ethereum's EVM, Solana's runtime with its parallel execution based on transactions declaring which accounts they touch, the Move VM used by Aptos and Sui, and CosmWasm in the Cosmos ecosystem. Proving systems increasingly use general-purpose virtual machines based on RISC-V so that ordinary programs can be proven rather than rewritten as circuits.
In practice
Solana transactions declare in advance which accounts they will read and write, which lets the runtime execute non-overlapping transactions in parallel.
The common misunderstanding
A blockchain virtual machine is not a server a developer rents; it is a rule set that every node runs, and the same code executes on all of them.