PSSO (Pass Slipped Stitch Over)
PSSO means lifting a previously slipped stitch over a newly knitted stitch and off the needle, often combined with a slip stitch to form a decrease.
Explanation
PSSO (pass slipped stitch over) is often part of the decrease combination 'sl1, k1, psso': you slip a stitch without knitting it, knit the next stitch, and then lift the slipped stitch over the newly knitted one and off the needle. This produces a left-leaning decrease quite similar to SSK, and is common in lace and eyelet patterns where the pattern specifically calls for this combination to achieve a particular visual effect.