I feel like these terms need to be defined so that everyone is on the same page. As I understand them:

Minmaxing:
Maxing your primary attributes/gear/abilities at the cost of minimizing things you deem non-essential to your character's primary purpose. A minmaxed character should have a 17 in their primary, a 16 in their secondary, and 8 or 9 in ~two "dump" stats (with Point Buy). They have obvious and devastating weaknesses.

Powergaming:
Related to minmaxing, but more about Being Maximally Effective Overall than Being the Best at One Thing. Such a player will choose feats/spells/multiclassing to be the most effective character and focus on using optimal tactics while in combat. They *can and often will* minmax, but it's not a prerequisite. If only some players powergame, combats will be unbalanced and the DM will likely need to compensate somehow.

Munchkining:
The more advanced form of powergamer, who makes heavy use of unintended combinations via exploits, loopholes, and vagueness in the rules. A munchkin is a powergamer, but a powergamer isn't necessarily a munchkin.

Optimizing, or "Playing an Effective & Well-Rounded Character":
Basically what @Kanisatha describes above. Your primary stat is good (16 with Point Buy), your secondary stats are okay but not necessary amazing (~12-14), and you don't necessarily have multiple dump stats.

Playing a suboptimal, but still effective, character (usually for roleplay reasons):
Similar to above, but you *slightly* decrease some of your primary/secondary stats. The focus is more on creating the believable character that you want to roleplay as. I'd argue that there's still a minimum Primary Ability Score that you can have and still be in this category: 14. Anything lower, unless you prepare by e.g., only taking buff/area control spells, and you are...

Gimping your Character: ([In Solo Play] Doing a Challenge Run, or [In Multiplayer] Being an Ineffective Teammate)
Building a character that is so unoptimized that they don't pull anywhere near their weight and actively make it harder for the team to succeed. E.g., a Wizard with an Int of 10 or less; a Frontline Fighter with <10 Con; a Monk with <10 Wis. These characters will spend most of their time doing little damage, missing, dying, or dead.

Importantly, none of the above are *necessarily exclusive* with good roleplay. Powergamers can be good roleplayers while suboptimal-character players can be That Guy, and vice versa.