Example:
abandon
to leave someone, especially someone you are responsible for
what does "to" mean here?
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Sign up to join this communityDictionaries use the infinitive form both for the main word being defined and the words used to describe it.
abandon
is a "bare" infinitive (the to is omitted).
to leave
is a full 'to' infinitive. In a dictionary, it would be unhelpful for every verb to be preceded by "to", so it is omitted from the main word of each entry. But in full sentences forming a definition, it is added.
What does "to" mean? Nothing. It just marks the infinitive form in the same way that an inflection does in other languages (eg aimer in French or amare in Latin or Italian).