Digitization, Digitalization and Digital Transformation

Digitization

Pure Analog-to-Digital conversion (Gartner), e.g. Scanning a document to get a PDF.
Data itself is not changed. Merely transformed - encoded in digital format.
Digitization helps organizations when the digital data is used to automate. However, the process of digitization itself doesn't optimize any industrial process.
Examples from manufacturing - converting a mechanical (manual) measurement to e-read, physical 2D sheets to CAD drawings, etc.

Use of digital technologies to change the processes (Gartner).
A process of moving towards a digital business.
Using digital information technologies to optimize the processes - re-engineering, re-imagining the processes. Therefore, this is much more than digitization.
Examples from manufacturing: PLC logic or PID control in a microprocessor-based system, sequenced logic for a batch process, automated shutdown logic, etc. An error in a transmitter generating a work order in the ERP maintenance system for a maintenance tech.

If digitization is a conversion (of data), digitalization is a step further (for a processes).
One "digitizes" a document; but "digitalizes" a factory.

Profound and accelerating transformation of business activities, processes, competencies, and models to fully leverage the changes and opportunities brought by digital technologies and their impact across society in a strategic and prioritized way [IEEE Computer Society definition].
More than applying technology to existing business, it is the capacity to rapidly adapt when required through the intelligent use of technologies and information [iScoop].
Business transformation enabled by digitalization [Gartner].
Digital transformation is the changing of business processes enabled or forced by digitalization technologies.