Abbreviation for the s-row irregular conjugation (sa-hen). It denotes verbs in the s-row irregular group; the prototypical example is verbs using 'suru' (e.g., benkyou-suru, unten-suru).
「勉強する」はサ変動詞に属し、名詞＋するの形で動詞になります。
"Benkyou-suru" belongs to the sa-hen verbs: a noun + 'suru' becomes a verb.
文法ではサ変と呼んで、特殊な活用をまとめて説明します。
In grammar it's called 'sa-hen,' grouping together verbs with a special conjugation.