This term gets thrown around sometimes in biology articles, and I have a rough idea of what it is: during embryonic development, the tissue differentiates in a certain spatial configuration. Wikipedia also mentions that its two main mechanisms are regional specification and cell differentiation. However, I would really enjoy a thorough definition of the term (either in the form of an answer or a reference).
I would guess that the term "tissue patterning" could be an umbrella term for multiple different developmental processes. In precise (more modern) developmental terms it could be a old synonym or sub-type for "Regional differentiation" https://en.wikipedia.org/wiki/Regional_differentiation
It also fits to the concept of X-inactivation in female mammals, but this phenomenon can not be observed by the bald eye.
It is quite possible the term originated from pigmentation biology an early genetic discipline. The underlying mechanism are the Pibald mutations , which result in dysfunction in dorso-ventral melanoblast migration , a dominant mutation, that is a source of white spots and patches in many mammals.
These mutations are almost omnipresent in domesticated furry mammals (cows, dogs, cats rats,horses) as well as some wild species where it gives an evolutionary advantage. You can also check out cat coat genetics its a lot of fun and relative easy classical genetics.
Excurse biological definitions:
Many biological terms originated in the first years of biology as a science (<1820 ) when genetics, microbiology, evolution and developmental biology where not discovered as disciplines yet (Mendel, Pasteur, Darwin&Wallace where born later, Boviery, Speemann even later), so people (zoologists, botanists) just named the thing they observed, without knowing what the underlying mechanism was. So since they did not have the concept of mendelian inheritance, nor embryonic development, they just called it in layman's therms: "patterning".
Nowadays with the term "tissue patterning" is not totally clear anymore what an author means, if it has no proper context, in my opinion. We have clearer defined terms.