Testability

ISTQB Glossary definition 

The capability of the software product to enable modified software to be tested.     [ISO 9126] See also Maintainability.

In simple english,

  it is a measure of how much a software system is suitable for testing.

Field Notes:

• it is a measure that is mostly on a qualitative scale rather than a quantitative scale. 
• It is normal for this to be expressed as just as an Yes or No.
• Sanity test or smoke test is conducted to determine this.
• Most often this measure is found using a experienced test analyst or test lead to check the system.
• It is however, advised to have a list of test cases that can be used to determine testability.