A corporate tax haven is pretty much what most people would imagine it to be: a jurisdiction that provides facilities to help multinationals escape taxes elsewhere. More formally, we define it as

A jurisdiction that seeks to attract multinational companies by offering facilities that enable them to escape or undermine the tax laws, rules and regulations of other jurisdictions, reducing their tax payments in these jurisdictions.

Current data and research suggests that governments around the world lose over US$ 500 billion — half a trillion dollars — in tax each year due to corporate tax havens. The IMF,  for instance, recently estimated that rich countries lose some $450 billion annually to tax-haven related corporate tax dodging, while lower-income countries lose $200 billion (which represents a bigger share relative to their smaller economies).  But remember: tax losses are just one dimension of the damage. The harm that corporate tax havens inflict on democracy, on society, and on our trust in politics and markets, is incalculable.
Financial Secrecy Index – 2020 ResultsMake Taxes Work For Women