s.t 가 무슨 뜻인가요?

<DIV> <TABLE style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; WIDTH: 236px; BORDER-COLLAPSE: collapse; HEIGHT: 61px; BORDER-TOP: medium none; BORDER-RIGHT: medium none" class=txc-table border=0 cellSpacing=0 cellPadding=0 width=236> <TBODY> <TR> <TD style="BORDER-BOTTOM: #000 1px solid; TEXT-ALIGN: center; BORDER-LEFT: #0900ff 2px solid; BACKGROUND-COLOR: #e4f7ba; WIDTH: 233px; BORDER-TOP: #0900ff 2px solid; BORDER-RIGHT: #000 1px solid" rowSpan=2 rows="1" cols="1">Chapter</TD> <TD style="BORDER-BOTTOM: #000 1px solid; TEXT-ALIGN: center; BACKGROUND-COLOR: #e4f7ba; WIDTH: 233px; BORDER-TOP: #0900ff 2px solid; BORDER-RIGHT: #000 1px solid" rows="1" cols="1">편</TD> <TD style="BORDER-BOTTOM: #000 1px solid; TEXT-ALIGN: center; BACKGROUND-COLOR: #e4f7ba; WIDTH: 233px; BORDER-TOP: #0900ff 2px solid; BORDER-RIGHT: #0900ff 2px solid" rows="1" cols="1">3</TD></TR> <TR> <TD style="BORDER-BOTTOM: #000 1px solid; TEXT-ALIGN: center; BACKGROUND-COLOR: #e4f7ba; WIDTH: 233px; BORDER-RIGHT: #000 1px solid" rows="1" cols="1">장</TD> <TD style="BORDER-BOTTOM: #000 1px solid; TEXT-ALIGN: center; BACKGROUND-COLOR: #e4f7ba; WIDTH: 233px; BORDER-RIGHT: #0900ff 2px solid" rows="1" cols="1">7</TD></TR> <TR> <TD style="BORDER-BOTTOM: #0900ff 2px solid; TEXT-ALIGN: center; BORDER-LEFT: #0900ff 2px solid; BACKGROUND-COLOR: #e4f7ba; WIDTH: 466px; BORDER-TOP: medium none; BORDER-RIGHT: #000 1px solid" colSpan=2 rows="1" cols="1">Page</TD> <TD style="BORDER-BOTTOM: #0900ff 2px solid; BACKGROUND-COLOR: #e4f7ba; WIDTH: 233px; BORDER-RIGHT: #0900ff 2px solid" rows="1" cols="1">145</TD></TR></TBODY></TABLE></DIV> 정병열 7급 경제학 3판   서브노트에   예산제약하에서의 효용극대화 Max U(X,Y) = X*Y s.t. Px*X+Py*Y=M   이라 되어 있던데 s.t. 가 무엇의 약자인가요? ^^;;;