The reduction (special fiber) $E_{\mathfrak p}$ of (the Neron model of) an elliptic curve $E$ modulo a prime ${\mathfrak p}$ is one of:
1. good reduction = stable reduction = $E_{\mathfrak p}$ is non-singular
2. multiplicative reduction = semi-stable reduction = $E_{\mathfrak p}$ is a product of the multiplicative group times a finite group
3. additive reduction = unstable reduction = $E_{\mathfrak p}$ is a product of the additive group times a finite group