What is the term for when you become more aware of something?

For example, when you buy a car, you start becoming more aware of cars with a similar make and model. The number of that type of car hasn't increased, but your awareness of it has.

Similarly, when you learn a new word, you start hearing the same word used in different places. You probably just filtered the word out previously, but now you've become aware of it.

How do you describe the phenomenon when you become more aware of something after an initial or significant exposure or experience with that thing? What term or word can you use?

It is called the "Baader-Meinhof Phenomenon" or "frequency illusion".

Baader-Meinhof is the phenomenon where one happens upon some obscure piece of information-- often an unfamiliar word or name-- and soon afterwards encounters the same subject again, often repeatedly.

–"The Baader-Meinhof Phenomenon," written by Alan Bellows

Stanford linguistics professor Arnold Zwicky coined [the term "frequency illusion"] in 2006 to describe the syndrome in which a concept or thing you just found out about suddenly seems to crop up everywhere. It’s caused, he wrote, by two psychological processes.

The first, selective attention, kicks in when you’re struck by a new word, thing, or idea; after that, you unconsciously keep an eye out for it, and as a result find it surprisingly often.

The second process, confirmation bias, reassures you that each sighting is further proof of your impression that the thing has gained overnight omnipresence.

–"There's a Name for That: The Baader-Meinhof Phenomenon," by Pacific Standard staff

Additional source:

"Just Between Dr. Language and I," by Arnold Zwicky on Language Log

Note: This also looks like a question for:

https://cogsci.stackexchange.com/questions/tagged/cognitive-psychology

I've called it red car syndrome for decades, but apparently to the world at large it's...

Blue Car Syndrome - a whole article there, but it's perfectly well defined in UrbanDictionary as...

The act of seeing or hearing something and then suddenly seeing it everywhere.
You buy a blue car, and suddenly you notice blue cars everywhere.

Technically, as ermanen notes, it's a type of cognitive bias called the Frequency Illusion.