What is a defensive action in baseball?

Answer 1

That is a term generally used when a base runner is stealing a base. The catcher will not attempt to make a play on the runner (throw the ball to the base that the runner is attempting to steal). Sometimes, the official scorer will rule this 'defensive indifference' and not award the runner with a stolen base. You might see this in the 9th inning of a game where the team batting is down by several runs and the defensive team is more concerned with getting the batter out than attempting to throw out a base runner attempting to steal since that run will not affect the outcome of the game.

Answer 2

Defensive indifference:

The official scorer shall not score a stolen base when a runner advances solely because of the defensive team's indifference to the runner's advance. The official scorer shall score such a play as a fielder's choice.
Rule 10.07(g) Comment: The scorer shall consider, in judging whether the defensive team has been indifferent to a runner's advance, the totality of the circumstances, including the inning and score of the game, whether the defensive team had held the runner on base, whether the pitcher had made any pickoff attempts on that runner before the runner's advance, whether the fielder ordinarily expected to cover the base to which the runner advanced made a move to cover such base, whether the defensive team had a legitimate strategic motive to not contest the runner's advance or whether the defensive team might be trying impermissibly to deny the runner credit for a stolen base. For example, with runners on first and third bases, the official scorer should ordinarily credit a stolen base when the runner on first advances to second, if, in the scorer's judgment, the defensive team had a legitimate strategic motive-namely, preventing the runner on third base from scoring on the throw to second base-not to contest the runner's advance to second base. The official scorer may conclude that the defensive team is impermissibly trying to deny a runner credit for a stolen base if, for example, the defensive team fails to defend the advance of a runner approaching a league or career record or a league statistical title.