Statutory definition
Constructive dismissal
Also: forced resignation
Definition
Where an employee resigns in response to a repudiatory breach of contract by the employer, the resignation is treated as a dismissal by the employer. The employee must not delay so long as to affirm the contract.
In plain English: You quit, but only because your employer made the job impossible — so the law treats it as if they dismissed you.
You did not name a jurisdiction, so this shows the GB definition. The term is defined differently elsewhere — see below.
Usage
Resigning first and looking for the breach afterwards rarely succeeds. The breach must be the reason for leaving, and delay can defeat the claim.