A semistable degeneration \(X\to {\mathbf{A}}^1\): a proper morphism where the fibers away from zero are smooth and the fiber over zero is an SNC divisor in the total space: 
A semistable degeneration \(X\to {\mathbf{A}}^1\): a proper morphism where the fibers away from zero are smooth and the fiber over zero is an SNC divisor in the total space: