The act of deliberately stalling, delaying, or preventing legislation from being passed. It has a negative connotation, as politicians do not want to be seen as preventing progress.
Obstructionist politicians are typically either a a party with control of one branch or house of a legislature, or part of a minority party with enough of a plurality to prevent legislation (like through a filibuster)
Obstructionism is often a useful tool for an opposition party. Politico: “Still, for the most part, obstructionism worked. Americans always tell pollsters they want politicians to work together, but as Washington Democrats decide how to approach the Trump era from the minority, they will be keenly aware that the Republican Party’s decision to throw sand in the gears of government throughout the Obama era helped the Republican Party wrest unified control of that government.”