There are definitions in the excellent website, www.mentomusic.com.

"What is mento? Here's a short answer: It's a Jamaican music that is largely unknown outside of Jamaica that is the grandpappy of regfgae. For a ska or reggae fan, mento music sounds familiar and exotic and unfamiliar.... "
Following this is a longer definition.
"Mento music had its beginnings in Jamaica in the 19th century, and was uniquely Jamaican fusion of African and European musical traditions.........
"It wasn't until the erly 1950s that true mento recordings first began to appear [the pre-history is discussed, I have skipped that].
"These recordings reveal mento to be a diverse musical genre, sometimes played with reckless abandon and others with orderly precision.... In addition to mento's African and European roots, by this time it had also encompassed pan-Carribean influences, as well as American jazz."
"During this time, Trinidadian calypso was the Caribbean's top musical export, and the term "calypso" was used generically applied to Jamaican mento as well.....
"Adding to the confusion, Jamaica had its own calypso singers that did not record mento, such as Lord Creator. ....and mento artists would often perform alypso songs in the mento style.... Some mento artists followed the calypsonian practice of adding a title such as "Lord" to their name.

"But make no mistake, mento is a distinctly different sound from calypso, with its own instrumentation, rhythms, pacing, vocal styles, harmonies and lyrical concerns."

Political content entered some of the songs, particularly those with a fusion of styles.

The above sounds confused and there is no clear explanation, except to listen to the music and note its differences.

I like to listen to it, but I don't know anywhere near enough to offer a useful definition.