More Formal:
* Technical requirements document
* Functional specification
* Design specification
* System specification
* Product specification
* Engineering specifications
* Technical data sheet
* Performance specifications
* Technical documentation
* Technical description
More Informal:
* Specs
* Technical details
* Requirements
* Blueprints
* Guidelines
* Design document
* Engineering drawings
The best alternative will depend on the specific situation and the level of formality required.
For example, if you're discussing a complex software system, "functional specification" might be the most appropriate term. However, if you're simply talking about the basic features of a product, "specs" might be sufficient.