Law 12 of the FIFA Laws of the Game (LOTG) specifies the following:
"An indirect free kick is also awarded to the opposing team if a player, in the opinion of the referee:....
• impedes the progress of an opponent"
This is what is commonly referred to as "obstruction". The "impedes" terminology has been the official wording for the entire 12 years I have been involved with the game.
The LOTG do not explain it any further than this, but the typical instruction to referees is that a player cannot move into a position which impedes an opponent unless they are within "playing distance" of the ball and playing the ball by shielding it. They do not have to actually touch the ball. What constitutes "playing distance" is in the judgement of the referee depending on age, ability, and skill level of the competition. Playing distance for U18 boys, for example, would clearly be more than playing distance for U13 girls.
