I know that technique is how the singer control her/his voice to make a certain musical effect, but what confuses me are words like 'polished technique' and 'solid technique' ; so what do these words exactly mean?
A “technique” is (in general) a practical method to achieve something (such as an musical effect when singing) which requires skill.
The “technique”, however, is the mastery of these methods and practical skills in a particular field (such as operatic singing).
So when you say about a singer that they have a “solid technique”, I would understand that they have mastered the basics of operatic singing very well. A singer with a “solid technique” would have a beautiful, consistant vibrato, e.g.
If you say that their technique is “polished”, I would understand that it goes further than just “solid”. I would understand that they have mastered their practical skill up to subtlety. A singer with a “polished technique” would probably not use the same vibrato depending on the repertoire they are singing: you don’t sing baroque as you sing bel canto.
I would just call it 'reviewer-speak', not an absolute reference.
An opinion that feels like it needs more than one word to describe it.
Like with advertisers selling soap, 'clean' is just not enough. Their product needs to get things 'squeaky clean' or 'touchably soft' etc etc.
Technique seems to demand a qualifier, good/bad/average... solid/polished.
They are degrees of accomplishment, but not absolutes.
Good, bad & average are qualifications, but like 'clean', they're just not enough to engage the audience, so artistic license is employed & we increase the excitement of our review by 'advertising' our opinion, & can now employ poetic license, comparing the workmanlike 'solid' to the far more accomplished 'polished'.
What does the word technique mean in Opera singing?
Breath support, mouth shape to get the vowels right, and diction.
The singer with a polished technique has mastered the basics, and is able to do some sophisticated things with his/her voice.
There are no defects in this singer's range, no problem areas. This singer understands diction. This singer doesn't necessarily make you cry, but he or she does everything correctly, and has trained his or her voice long enough that his voice is responding reliably to the various instructions his brain sends to it.
I'm not saying it's a bad thing to have a solid technique -- it's actually a very good thing -- but to be a great singer, having a solid technique isn't quite enough.