Design Thinking
The process and methodology to approach or solve a problem by understanding human needs, and reframing the problem in human-centric ways.

Empathise: Understanding the human needs involved.
Setting aside designers’ , engineers’ and company’s needs and assumptions, immersing into
the actual context of the problem; observing people actual behaviours, experiences and motivation.
Gathering as much data as possible about users and their needs.
Define: Re-framing and defining the problem in human-centric ways
Analysing and understanding data and informations gathered during the empathise stage and synthesize them in order to define the core problem.
Ideate: Creating many ideas in ideation sessions.
Based on what has been defined in previous stages, ideation process can now be started. 
Brainstorming sessions, thinking outside the box, looking for alternative ways to solve the problem are all methodologies to narrow possibilities and finding the right path.
Prototype: Building an inexpensive version of the product.
A prototype is the inexpansive and scaled down version of a product of a feature we want to build. This saves money and time compared to building the actual product, and allows testing and validation before putting financial and human resources that may be wasted without testing.
Test: Observation and evaluation of the idea and prototype
This is the process of evaluating what has been built in the prototype stage. Prototypes are usually almost fully functional, so that users can actually try and test the product/feature. 
Observing and analysing users behaviour helps to understand if we are on the right path or we need to get back to ideation stage and think differently.
The linear representation of the process gives an easier understanding of the stages, but this is an ongoing, flexible and non linear process, where stages can be run at different pace and in different phases, and where data and informations can be gathered throughout the entire process.

Resources
Interactive Design Foundation: Design Thinking
Careerfoundry: Key Principles of Design Thinking