Metrics matter. Your product’s success is dependent on and defined by metrics. Metrics are a lens into your product’s health and performance. Defining your metrics, therefore, should be as intrinsic to your product development process as defining requirements.

What a metric is

In the broadest terms, a metric is any collectable, quantifiable measure that enables one to track the performance of an aspect of your product or business over time.

“…collectable, quantifiable measure…” This means a metric must be a value you can somehow collect by instrumenting your product.
“…track the performance…” You can measure a lot of dimensions of your product, but not all of them are tell-tale of its performance. Metrics must measure components of your product that are relevant to the product’s success (more on this in a subsequent part of this series).
“…an aspect…” It’s important to note that rarely does one single metric give you a sufficient look into the health of your product. A product will typically have multiple metrics to represent its various aspects, and this suite of metrics together paints a complete picture.
“…over time.” Metrics should capture changes in your product over time to help you set baselines with past data, to deduce trends, and to enable projections.