tenacious
"Tenacious" is a powerful adjective that most learners encounter when describing people who simply refuse to give up. It has a couple of closely related senses — a literal physical one and a more common figurative one about personality and determination.
When you describe a person as tenacious, you mean they hold on to their goals with incredible strength and do not stop, even when things get very hard. Think of a dog that grabs a rope and will not let go — that is the image behind this word. It is the most common way you will see 'tenacious' used.
everyday language, work, sports · Modern, widely used · figurative
In a more literal sense, tenacious describes something that grips or sticks very tightly and is hard to remove or shake off. Imagine ivy growing on a wall — its roots hold on so firmly that pulling it off is nearly impossible. That grip is tenacious.
everyday language, science, nature · Modern, also found in older texts
You can also use tenacious to describe someone who holds on to their ideas, beliefs, or positions very strongly and does not change them easily. It is similar to the 'determined' sense, but focused specifically on opinions and convictions rather than goals.
politics, debate, everyday language · Modern, widely used · figurative