What is a word?
The dictionary definition of a word is:
one of the units of speech or writing that native speakers of a language usually regard as the smallest isolable meaningful element of the language.
In most written languages, words are separated by spaces so it’s easy to see where each word begins and ends. In spoken language however, words are uttered in a more or less continuous stream and we mentally insert the gaps between the words. If you listen to an unfamiliar language, you are probably unable to separate the sounds you hear into individual words. As you learn a language your ability to ‘hear’ to individual words in speech gradually improves.
I think that written language shapes our perceptions of spoken language, at least to some extent. In some languages, such as German and Dutch, words are often glued together to make long compound words, e.g. Donaudamfschifffahrtskapitaen. If you speak on of these languages but can’t read or write it, you may perceive such compounds as separate words. In other languages, such as Chinese and Vietnamese, every syllable is written separately, which gives you the impression that such languages are monosyllabic, when in fact they do have many multi-syllable words.