From US LAcrosse

3. ASSIST: An assist is a play made by a player to her teammate who then scores a goal without having to evade excessive defensive pressure other than the goalkeeper. Only one assist per goal may be recorded.
The guideline to award an assist includes two parts: there must be a pass made to the recipient; and the recipient must take a shot. If the passer (potential assister) is able to move the ball to a teammate (recipient), who has maneuvered for a good or more advantageous position to take a shot, then the passer can be awarded with an assist. If the shooter, after receiving the ball from her teammate (passer/potential assister) must outrun or maneuver around any excessive defensive pressure before shooting, no assist can be awarded. It is not necessary for the shooter to possess the ball for a specific length of time, nor must the shooter limit the number of steps taken after she receives the ball for a shot. The only deciding factor, after the recipient receives a pass from her teammate, is the amount of pressure the recipient (shooter) had to evade to put her into a position for the shot, if any.