Hyperplasia

Hyperplasia means more cells; hypertrophy means bigger cells. Applied to muscle, hyperplasia would mean training produces additional muscle fibres rather than thickening the ones you already have.

It matters because whether it happens in humans is genuinely unresolved. It has been demonstrated in some animal models under extreme stretch-overload protocols, but human evidence is scarce and contested, partly because counting fibres in a living person is close to impossible. The consensus is that hypertrophy accounts for essentially all practical muscle growth in humans.

In practice this changes nothing about how you should train. Nobody has shown a protocol that reliably produces hyperplasia in people, and 'hyperplasia training' claims should be treated as marketing.