Explanation:
Namespace is a grouping or a container for a CloudWatch metric. The names must be valid XML characters, typically containing the alphanumeric characters "0-9A-Za-z" plus "."(period), "-" (hyphen), "_" (underscore), "/" (slash), "#" (hash), and ":" (colon). All AWS namespaces follow the convention AWS/<service>, such as AWS/EC2 and AWS/ELB.
Reference: http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/cloudwatch_concepts.html