When is 'the' needed before the word 'last'? For example, is it needed in the following sentence?

In (the) last meeting, we discussed the relationship between humidity and cloud formation.

Thanks in advance!
I think you need "the last".
You can leave out "the" (and it's usually left out, I think), in sentences like:

(The) Last time I saw her she was with another guy.
Emotion: smile
The definite article "the" is used with most nouns whose specific identity is known to the reader. In your example, both the writer and the reader evidently were at the last meeting, so they know which meeting they are talking about. If the writer were talking about discussing humidity and cloud formation at some future meeting, but was not clear on exactly which meeting that would be, he might write, "Let's discuss that at a future meeting." In this case, he would not use "the" because the specific identity of that future meeting is not known.