user1468: I occasionally see annotations/commentary where a move / line is said to make (for example) white win "by force". I am unsure if it has the general sense of "this move initiates a forcing sequence where white will come out on top" or something more specific, say, "with this move, white exerts indefensible pressure ('force') on a particular square / sector (say more attackers than defenders on f7 with further defense impossible) that will give them victory."
What exactly does it mean?

user70: It's a bit of hyperbole to be sure.  I can't think of many cases where such a phrase would be used.  Normally, "...and wins" is good enough.
But it means there's no conceivable way that White will lose the game and it takes no special skill to finish the game. The opposite phrase is Black's position is "dead lost."
I have also seen in print "black should resign" when the position was embarrassingly untenable but Black played on.

user2042: 
I occasionally see annotations/commentary where a move / line is said
  to make (for example) white win "by force". I am unsure if it has the
  general sense of "this move initiates a forcing sequence where white
  will come out on top" or something more specific, say, "with this
  move, white exerts indefensible pressure ('force') on a particular
  square / sector (say more attackers than defenders on f7 with further
  defense impossible) that will give them victory."
What exactly does it mean?

The bold part is correct answer. 
Forced move is exactly that: A move you are forced to play. A forcing move is exactly that: a move that forces the opponent to respond in a specific way, limiting his freedom of choice.
Take Alekhine's defense as example. White is forcing the exchange of knights, while Black is forced to exchange the knight:
[Title "Demonstrating difference between forced and forcing move"]
[fen ""]

1.e4 Nf6 2.e5 Nd5 3.c4 Nb6 4.c5 Nd5 5.Nc3 Nxc3 6.dxc3

The italic part in my quote of your question is never called "force". The term pressure is commonly used to describe it, like in this example:
[Title "White to move"]
[fen "r4r1k/pp2qppp/6b1/8/2B5/1QP5/PP3RPP/5R1K w - - 0 1"]

So in this example we can say something like this:
White exerts decisive pressure on the f7 square which would lead to a forced win for him if he plays 1.Bxf7.
After 1.Bxf7 Black is forced to exchange off all his pieces (not exchanging proves even worse for Black) after which the endgame is won for White. That is why we say White has a forced win: he can force Black to enter into a lost endgame.
Hopefully this clarifies things.

user2001: "Wins by force" means that the move leads to a series of moves that makes it impossible for the losing side to defend the position. Here, the definition of "winning" can vary between annotators. Checkmate is the extreme case of winning. Having an advantage of two pawns without any compensation for the opponent is somewhere at the low end of a winning position. The whole term "by force" refers to the fact that the opponent is unable to avoid getting a worse / losing position. At what point does a position become winning? That can depend on the position and the players involved. Usually, higher rated players have higher expectations on them to win better positions. If a game is annotated as "this move wins by force" then it means that the annotators expects the player at the board to win the position that arises after the sequence of moves is completed.

1468: <p>I occasionally see annotations/commentary where a move / line is said to make (for example) white win "by force". I am unsure if it has the general sense of "this move initiates a forcing sequence where white will come out on top" or something more specific, say, "with this move, white exerts indefensible pressure ('force') on a particular square / sector (say more attackers than defenders on f7 with further defense impossible) that will give them victory."</p>

<p>What exactly does it mean?</p>
 70: <p>It's a bit of hyperbole to be sure.  I can't think of many cases where such a phrase would be used.  Normally, "...and wins" is good enough.</p>

<p>But it means there's no conceivable way that White will lose the game and it takes no special skill to finish the game. The opposite phrase is Black's position is "dead lost."</p>

<p>I have also seen in print "black should resign" when the position was embarrassingly untenable but Black played on.</p>
 2042: <blockquote>
  <p>I occasionally see annotations/commentary where a move / line is said
  to make (for example) white win "by force". I am unsure if it has the
  general sense of <strong>"this move initiates a forcing sequence where white
  will come out on top"</strong> or something more specific, say, <em>"with this
  move, white exerts indefensible pressure ('force') on a particular
  square / sector (say more attackers than defenders on f7 with further
  defense impossible) that will give them victory."</em></p>
  
  <p>What exactly does it mean?</p>
</blockquote>

<p>The bold part is correct answer. </p>

<p>Forced move is exactly that: A move you are forced to play. A <strong>forcing</strong> move is exactly that: a move that forces the opponent to respond in a specific way, <strong>limiting his freedom of choice.</strong></p>

<p>Take <em>Alekhine's defense</em> as example. White is <strong>forcing</strong> the exchange of knights, while Black is <strong>forced</strong> to exchange the knight:</p>

<pre><code>[Title "Demonstrating difference between forced and forcing move"]
[fen ""]

1.e4 Nf6 2.e5 Nd5 3.c4 Nb6 4.c5 Nd5 5.Nc3 Nxc3 6.dxc3
</code></pre>

<p>The italic part in my quote of your question is never called "force". The term <em>pressure</em> is commonly used to describe it, like in this example:</p>

<pre><code>[Title "White to move"]
[fen "r4r1k/pp2qppp/6b1/8/2B5/1QP5/PP3RPP/5R1K w - - 0 1"]
</code></pre>

<p>So in this example we can say something like this:</p>

<p><em>White exerts decisive <strong>pressure</strong> on the <code>f7</code> square which would lead to a <strong>forced</strong> win for him if he plays <code>1.Bxf7</code>.</em></p>

<p>After <code>1.Bxf7</code> Black is <strong>forced</strong> to exchange off all his pieces (not exchanging proves even worse for Black) after which the endgame is won for White. That is why we say White has a forced win: he can <em>force</em> Black to enter into a lost endgame.</p>

<p>Hopefully this clarifies things.</p>
 2001: <p>"Wins by force" means that the move leads to a series of moves that makes it impossible for the losing side to defend the position. Here, the definition of "winning" can vary between annotators. Checkmate is the extreme case of winning. Having an advantage of two pawns without any compensation for the opponent is somewhere at the low end of a winning position. The whole term "by force" refers to the fact that the opponent is unable to avoid getting a worse / losing position. At what point does a position become winning? That can depend on the position and the players involved. Usually, higher rated players have higher expectations on them to win better positions. If a game is annotated as "this move wins by force" then it means that the annotators expects the player at the board to win the position that arises after the sequence of moves is completed.</p>
