A standard of measurement, or a system used to determine measurements. In basketball, a metric is typically associated with individual player or team statistics.
The terms "metric" and "statistic" can be used interchangeably in most instances, but there is also an inherent difference. The term "statistic" has been used in basketball since the early days of hoops competition. Field Goal Percentage, Free Throw Percentage and Points Per Game are all examples of basketball statistics. The term "metric" really came into use when advanced statistical analysis arrived on the scene. Offensive Rating, Win Shares and Effective Field Goal Percentage are all examples of basketball metrics.
So, even though "metric" and "statistic" are closely related, they are usually used differently when it comes to NBA basketball. Basic boxscore-type numbers are generally referred to as "statistics", while the more advanced analytics are typically called "metrics".