Is there a single, opposite word to “alibi”?

Solution 1:

proof
evidence or argument establishing or helping to establish a fact or the truth of a statement.

Here's the proof (you've been asking for) that I was at a dinner party last night.

This answers the OP example: For instance, I want to prove that I was "there" at a dinner party last night, not at another or other places, or I want to show off a picture taken at a dinner show together with movie stars..

While I agree with Josh61 that proof is not the antonym for alibi, you wouldn't say about a photo which testifies your presence at a dinner party as: "This is my alibi..."

Solution 2:

If an alibi is "exculpatory evidence", the its opposite would be "damning evidence"; the English idiom which more concisely communicates the idea of damning evidence is smoking gun.

From Wikipedia, for example:

The term "smoking gun" was originally, and is still primarily, a reference to an object or fact that serves as conclusive evidence of a crime or similar act. In addition to this, its meaning has evolved in uses completely unrelated to criminal activity: for example, scientific evidence that is highly suggestive in favor of a particular hypothesis is sometimes called smoking gun evidence. Its name originally came from the idea of finding a smoking (i.e., very recently fired) gun on the person of a suspect wanted for shooting someone, which in that situation would be nearly unshakable proof of having committed the crime. A piece of evidence that falls just short of being conclusive is sometimes referred to as a "smoldering gun."

So the power of idiom is derived from its clear imagery.

Solution 3:

The opposite of alibi, if there is one, surely is in flagrante delicto, meaning in the very act of wrongdoing (especially in an act of sexual misconduct). There are difficulties with this since, as you observe, alibi is a noun and this expression is adverbial.

But it would appear to me that proof of your attendance at the dinner party (as someone observed) constitutes an alibi. The worst-case opposite of that would have been to have been caught in flagrante delicto.

Solution 4:

Well, an alibi is said to be "exculpatory" (taking away blame), so if "culpatory" is a word, it would mean "attaching blame to".

Other adjectives include "damning" or "incriminating" (evidence), which helps to establish "culpability".

If the perpetrator was caught in the act, he is said to have been "caught red-handed". The cops could be said to "have the goods on him". If the evidence is strong, prosecuting attorney has an "ironclad case" or an "open-and-shut case" against him. The evidence might be said to be "incontrovertible" (cannot be disproved).

A related term is "corpus delicti", which refers to proving that a crime has been committed (you can't be proven guilty of a crime unless prosecutors can establish that a crime has been committed.)

But none of these is an actual one-word noun opposite of "alibi".

Solution 5:

What you are seeking is a single-word noun that means "proof or evidence of presence".

I'm pretty sure that no such word exists in English.

However, given that alibi is a Latin word, perhaps we can at least invent a corresponding Latin expression that means "indication of presence".

Accordingly, I propose the term indicium praesentiae. Admittedly, this isn't the type of expression most people would want to drop into a casual conversation, but I can see it having its uses in the courtroom, medicine or other specialized contexts.