Rate of convergence of a sequence in $\mathbb{R}$ and Big O notation From Wikipedia

$f(x) = O(g(x))$ if and only if there exists a positive real number
  $M$ and a real number $x_0$ such that $|f(x)| \le \; M |g(x)|\mbox{
 for all }x>x_0$.

Also from Wikipedia

Suppose that the sequence $\{x_k\}$ converges to the number $L$. We
  say that this sequence converges linearly to $L$, if there exists a
  number $μ ∈ (0, 1)$ such that $\lim_{k\to \infty}
 \frac{|x_{k+1}-L|}{|x_k-L|} = \mu$.
If the sequences converges, and
  
  
*
  
*$μ = 0$, then the sequence is said to converge superlinearly.
  
*$μ = 1$, then the sequence is said to converge sublinearly.
  

I was wondering


*

*Is it true that if $\{x_n\}$ either linearly, superlinearly or sublinearly
converges to $L$,  only if  $|x_{n+1}-L| = O(|x_n-L|)$? This is based on
what I have understood from their definitions and viewing $\{ x_{n+1}-L \}$ and $\{ x_n-L \}$ as functions of $n$. Note that "only if" here means "if" may not be true, since $\mu$ may lie outside of $[0,1]$ and $\{x_n\}$ may not converge.

*Some Optimization book says that the steepest descent algorithm
has linear rate of convergence, and writes $|x_{n+1}-L| =
    O(|x_n-L|)$. Is the usage of big O notation here expanding the meaning of linear rate of convergence?


Thanks and regards!

To answer your added question,
from the definition,
$x_n$ converges to $L$ if and only if
$|x_n-L| \to 0$ as $n \to \infty$.
The existence of a positive c such that
$c < 1$ and $|x_{n+1}-L| \le c|x_n-L|$
is sufficient for convergence, but not necessary.
For example, if $x_n = 1/(\ln n)$,
then $x_n \to 0$, but there is no $c < 1$
such that $x_{n+1} < c x_n$ for all large enough n.
It can be shown that there is no slowest rate of convergence - 
for any rate of convergence, a slower one can be constructed. 
This is sort of the inverse of constructing 
arbitrarily fast growing functions
and can lead to many interesting places.
