Using auxiliary with past participle

I was searching on the meaning of skirt in Longman dictionary and I found the definition which is: "a piece of outer clothing worn by women and girls", Why did not he say: "a piece of outer clothing is worn by women and girls", Why did they not use an auxiliary verb?

Solution 1:

"a piece of outer clothing worn by women and girls" is not a complete sentence. In a sentence it would be:

A skirt is a piece of outer clothing worn by women and girls

Past participles can be used together with an auxiliary verb to form a sentence or without one and then they act as adjectives.

It's the same as saying:

a piece of outer clothing that is worn by women and girls