Software metrics are quantitative data related to software.
A software metric is an objective and reproducible quantitative measure of some property of the product, its source code, its specification, or the development process that was applied. These can refer to development time metrics such as code-coverage or runtime metrics such as response-time.
Although the generic term "metrics" is usually used to refer to the latter especially in server-side development. For further information about metrics and their importance in modern software development you can read the seminal work Measure Anything Measure Everything by Ian Malpass.
This tag should be applied to general questions about metrics or questions that span multiple metrics types or implementations or if your are not sure which specific tag applies. For questions specific to a particular metric system more specific tags such as graphite,statsd,codahale-metrics and so forth.