Basic Trope: A character starts a new chapter of life as his/her children have grown up and left the house.
Straight: Alice and Bob have three adult children: Alan, Billy, and Carol. As soon as Carol (the youngest child) leaves home, Alice and Bob have no idea what to do with their newfound free time.
Once the last child has moved away, Alice and Bob suddenly feel that their lives are meaningless.
Alice thinks this way when the children are merely visiting their aunt for one weekend.
Downplayed: Some days Alice misses the kids, other times she doesn't.
A large part of Alice's identity is simply being the mother. Now that the kids are not kids anymore and don't need her, she feels like she's lost that sense of purpose.
Alice misses the kids, and the happy memories — and maybe those memories that weren't so happy at the time (such as when Billy discovered his artistic side and the newly-painted wall), but are now something to look back on and laugh at.
Lampshaded: "It's almost too quiet without the kids here."
The youngest of Alice's children has finally gone off to college...and made new friends and found an apartment to live in.
Billy is away at college and dating a Hard-Drinking Party Girl...and spent all his money on her. He tries to make Alice and Bob miss him so they'll worry and send him more money (and not fret about his appalling GPA).
Exploited: An apartment or condo complex markets itself to older adults whose children have grown up, now that they have less (or no) need of all that extra space.
Alice and Bob take full advantage of their newfound freedom. First, they turn their children's old bedrooms into a computer room, a home gym, and a den, respectively. Then they take up new hobbies, go on dream vacations together, and do any other things they wanted to do before, but couldn't with Children in Tow.
Alice talks the kids into moving back.
Discussed: "So, Bob, what's it like not having those kids around? Do you miss them?" "Actually, no. Alice and I have a lot more space now, not to mention free time."
Conversed: "Why do Alice and Bob miss their children so much? As I seem to remember from earlier seasons of this show, parenting was exceptionally stressful for them."
Implied: Alice sometimes looks into her childrens' old bedrooms for a few wistful seconds.