I'm a beginner at a guitar so I'm not too familiar with the terminologies. I know that a riff is the structural part of the song, the one that makes it recognisable. But what do you call the strumming of chords. Is it a melody?
If you are talking strictly about the rhythm the chords are strummed it is simply the Strumming pattern. A riff can be full chords, single notes back to back, several notes at a time, or any combination of all of those. Riffs can last through the entire song, only happen between parts that contain the melody or can even sometimes contain the melody within the riff.
If you mean how quickly the chords are changing it is called the harmonic rhythm.
Melody is the notes sung by the singer, or played by the lead instrument, and usually consists of one note at a time.
If it isn't the 'strumming pattern' then it's the 'rhythm pattern'. A pattern is something repeated, and the rhythm of the chords being played is the rhythm pattern. Usually played by the rhythm guitarist. Not often called the strummer!
One dictionary definition of 'strum' is - to play (a stringed or keyboard instrument) especially carelessly or unskilfully. Take it as you will...
Strumming is not melody. Melody refers to the "main theme" of the song, in Jazz it's called the head. The melody is that part that makes the song recognizable. In a moder rock or pop band it would be the part the signer sings. Strumming is done (by the guitarist) to add interesting rhythm to the supporting parts of the song.
There are different strumming techniques, and rhythmic patters and those may have specific names.
It can get confusing since in modern music the "recognizable part" may be played by the rhythm section as well as the singer. Case in point Good Times Bad Times by Zep. The melody is expressed in the lyric "In the days of my youth I was ..." etc. But the opening guitar riff is very recognizable, and a part that everyone who has heard the song can identify in a second.
Also, I have heard the term "Riff" referred to both rhythmic patterns and melodic solo patterns. Like the comments state, it's a repeated pattern. This can be a strummed chord riff, or a melodic phrase that is repeated in a solo.
In the Mel Bay guitar series each new "chapter" or section (which is typically a new key) includes exercises on such types of patterns to play for chord progressions. It's not very sophisticated but a starting point.