Igor wrote in [comment #5](https://forum.azimuthproject.org/discussion/comment/20805/#Comment_20805):

> In the definition of tensor product in Wikipedia it is unclear what do they mean by \$$e_i \otimes f_j\$$, where \$$e_i\$$ and \$$f_j\$$ are basis vectors. They define \$$\otimes\$$ for vector spaces, while not specifying what this means for individual vectors in these spaces.

They actually do explain this in the section [tensor product of vector spaces](https://en.wikipedia.org/wiki/Tensor_product#Tensor_product_of_vector_spaces): they say that if you choose a basis \$$e_i\$$ of the vector space \$$V\$$ and a basis \$$f_j\$$ of the vector space \$$W\$$, the vector space \$$V \otimes W\$$ has a basis consisting of meaningless formal symbols \$$e_i \otimes f_j\$$.

They don't actually say "meaningless formal symbols", but they say something that means the same if you're a mathematician: they say "each basis element is denoted \$$e_i \otimes f_j\$$". The word "denoted" here means "don't ask what it means: it's just a meaningless formal symbol".

There are dozens of ways to describe tensor products of vector spaces, and Wikipedia describes a few, but this is the quickest. Not the best, but the quickest.