Reading a book from 1995, I encountered the word lest in the proof of a theorem. I had not seen it in about twenty years of reading and working on mathematical contexts. Is this word used nowadays? Babylon suggested to use "against the possibility" instead of it but I found it very easy and beautiful. Did this online software suggest me correctly?
It's a conjunction mostly used in formal documents. The word originally comes from old English and means to prevent something undesirable from happening and in case.
She tiptoed lest her mother should hear her. Meaning in order to prevent here.
She worried lest she should be late. Meaning in case here.
A good alternative is in order to prevent and in case both used in different contexts.
Examples source Wordweb.
One good alternative is to use so as not to
Do not play with matches lest you (should) burn.
Do not play with matches so as not to burn.
The normal meaning of 'lest' is roughly "so that [it] does not happen", i.e. you do some precaution lest a consequence should happen.
I would say that its use is fairly literary, which may be why you did not encounter it before.
The word lest means "for fear that" or "so (not) that", and is actually an old contraction of a phrase which ended in "that". You can, for example, replace "lest we forget" with "for fear that we forget" or "so that we forget not" or "so that we [do] not forget".
"Lest" is a Conjunction and used in a formal register. Plus the fact that it employs the Subjunctive Mood makes it belong to an even smaller group.
Ex. She walked swiftly lest she be noticed.
You could substitute to this word: "to avoid (the risk of)" or, in other situations, "in case."
Long ago, I happen to encounter the phrase "for fear that," as the meaning of or in exchange for the word "lest." Ex. I feel pressured of being chosen for this position lest I may fall short of the expectations.