STRICT MEANING

Latin a ratione legis stricta, or stricta lege, Lat. ratio, « reason »; lex, « law »; strictus, « strict, narrow ».
Latin stricto sensu: Lat. sensus, « thought, idea, meaning ».
Latin ad litteram, littera « letter”.

The argument based on the strict meaning of a normative text prohibits restricting or extending the provisions of the law or regulation beyond what it clearly states. This can be considered as a special case of the principle « do not interpret what is clear »,  see legal arguments.
According to this principle, laws and regulatory provisions must be followed to the letter (ad litteram), in their strict sense (stricto sensu).

If the legal voting age to vote is 18 years, then you cannot forbid young people to vote on the day of their birthday because they are “just” 18, nor can you allow them to vote the day before their birthday because they are “almost” 18.
But from a linguistic point of view,

“he is almost 18 years old” is co-oriented with (is the same as) “he is 18 years old”,
“he is barely 18 years old” is co-oriented with (is the same as) “he is not 18 years old”.

The principle of interpretation stricto sensu cancels these co-orientations. The law establishes thresholds, and allows for threshold effects while almost and barely blur the boundaries. see orientation; orienting words.

The argument of the law’s generality holds that laws and regulations must be applied to all the concrete cases they cover. The strict meaning principle holds that the law must be applied to all such cases according to its literal meaning.

The strict meaning principle usually applies when the law is clear. When it seems clear to one judge but not so clear to her colleague, a stasis (a deadlock) emerges on the necessity of interpretation (distinct from a stasis of interpretation, where two interpretations clash).

The letter vs. the spirit of the law
In a legal context, the label ad litteram label refers to an argument that strictly follows “the letter” of the law, as opposed to its spirit. Interpretations that appeal “to the spirit” of the law may appeal to the legislative intent.
In an argumentative context, an ad litteram response addresses the strict letter of the opponent’s discourse, as opposed to its meaning as intended by the opponent. Ad litteram is then equivalent of ad orationem, see matter.