Spatial walk-off happens in birefringent crystals because an extraordinary beam's energy flows at a small angle to its wave vector, so it slides sideways away from the ordinary beam and the overlap is lost after a few millimeters. Temporal walk-off happens when femtosecond pulses at different wavelengths travel at different group velocities and separate. Both limit useful crystal length and are why ultrafast doublers are thin.
Walk-off in that BBO is 60 milliradians, so with a 50 micron spot you're done mixing after a millimeter.
Related terms
Phase matching
Keeping interacting waves in a nonlinear crystal in step so the generated light builds up instead of canceling.
SHG
Second Harmonic Generation: converting light to exactly double its frequency, half its wavelength, in a nonlinear crystal.
GVD
Group Velocity Dispersion: how strongly a material's group velocity changes with wavelength, which stretches short pulses.
Quasi-phase matching
Phase matching by periodically flipping a crystal's nonlinear orientation instead of relying on birefringence.