It could be stock you got in an employee stock option or stock ownership program.

FORBES: Insiders' Tax Break

Under terms of the Employee Stock Option Plan completed in April, which ceded 25.4% of the company to the pilots, the pilots can't sell shares until they quit.

FORBES: United We Fall

For instance, she says that firms are not buying back shares to finance employee stock-option programmes.

ECONOMIST: Share buybacks: A popular option | The

The fall in employee stock-option plans in America is reducing one pressure for buybacks, but the trend towards shorter-term investment horizons is perhaps increasing pressure from another quarter.

ECONOMIST: Buttonwood

Employers with publicly-traded stock almost always structure their 401k(s) to include company stock as an option and benefit when their stock rests in friendly employee hands.

FORBES: Flawed 401(k) Plan Structures to Blame for Systemic Failure

The idea is to hold on to employee stock until at least two years after an option is granted and one year after it's exercised.

FORBES: Out of Options