Portmanteau is a word that is formed by combining the sounds and meanings of two or more words. A very common example of a portmanteau is 'smog', a blend of 'smoke' and 'fog'.

In linguistics, it is called a blend, or a blend word, or lexical blend.

Some other examples:

Breakfast + Lunch = Brunch
Costume + Roleplay = Cosplay
Motor (Motorist) + Hotel = Motel
Web + Seminar = Webinar

Sometimes, people create a portmanteau by blending names of persons as well. A recent famous example of such blending is 'Bennifer' (Ben Affleck and Jennifer Lopez). While they famously dated in the early 2000s, a tabloid tagged them with this 'Bennifer' portmanteau. Later, they separated. After they got back together again in 2021, people started mentioning the couple as 'Bennifer 2.0'.