What does the ‘#’ character (number sign) mean in jQuery?

Now # would/could mean private instance fields: https://tc39.github.io/proposal-class-fields/

2021 update:

Class fields are public by default, but private class members can be
created by using a hash # prefix. The privacy encapsulation of these
class features is enforced by JavaScript itself.

class ClassWithPrivateField {
  #privateField;
}

class ClassWithPrivateMethod {
  #privateMethod() {
    return 'hello world';
  }
}

class ClassWithPrivateStaticField {
  static #PRIVATE_STATIC_FIELD;
}

class ClassWithPrivateStaticMethod {
  static #privateStaticMethod() {
    return 'hello world';
  }
}

source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields