Discrepancies in mathematical definitions. [closed]

What are some examples where there is a discrepancy in the mathematical definition of a term?

For example :

$\bullet$ Isosceles triangle: "exactly two sides are equal" or does it say "minimum two sides are equal"?

$\bullet$ Binomial Coefficient: $ {n \choose r} a^r \cdot b^{n-r}$ or $ {n \choose r} a^{n-r} \cdot b^r$

Are there any other such examples?

There are notations and terms in mathematics that have different conventions. For example, to some $\Bbb N$ is {$1,2,3,...$}, whereas to others $\Bbb N$ is {$0,1,2,3,...$}. For another example, some say a set is countable if there is an injective function from it to $\mathbb N$, whereas others say it has to be bijective. For another example, to some the dihedral group $D_n$ has $n$ elements, and to others it has $2n.$ In all of these cases, a writer using these should indicate which convention is being followed.