Definition
Key exchange is the procedure by which two communicating parties — neither of whom previously shared a secret — establish a shared cryptographic key. The classic algorithms are Diffie-Hellman (1976) and its elliptic-curve variant ECDH. RSA-based key transport, in which one party encrypts a key with the other's public key and sends it, is also a form of key exchange but is no longer preferred because it lacks Forward Secrecy.
Static vs Ephemeral
Static key exchange uses long-term keys directly: if those keys leak, every past session ever recorded becomes decryptable. Ephemeral key exchange (DHE, ECDHE) generates fresh key pairs for each session and discards them afterward, so a leak of the long-term identity key reveals nothing about past sessions. Modern VPN protocols (TLS 1.3, WireGuard, IKEv2 with PFS) all use ephemeral key exchange exclusively.
The Quantum Question
All currently deployed key exchanges — RSA, classical DH, ECDH — are vulnerable to a sufficiently large quantum computer. Such a computer does not yet exist, but adversaries can record encrypted traffic today and decrypt it later when one does. This 'harvest now, decrypt later' threat is driving deployment of post-quantum key exchanges (Kyber, X25519+Kyber hybrids) in TLS 1.3 and in some VPN protocols. The transition is in early stages and will accelerate over the coming decade.