der Sessel
armchair
A comfortable, upholstered seat for one person that typically has armrests.
Er saß gemütlich im Sessel und las ein Buch.
He sat comfortably in the armchair and read a book.

armchair / chair
Loading your saved state.
armchair
A comfortable, upholstered seat for one person that typically has armrests.
Er saß gemütlich im Sessel und las ein Buch.
He sat comfortably in the armchair and read a book.

Pay attention to the prepositions and the case. When you are moving toward the piece of furniture, it is 'in den Sessel' (into the armchair/chair, accusative). When you are already there, it is 'im Sessel' (in the armchair/chair, dative).
Setz dich doch in den bequemen Sessel am Fenster!
Go ahead and sit down in the comfortable armchair by the window!
In the standard meaning 'upholstered furniture', 'Sessel' (armchair) is often combined with adjectives that describe comfort or material. Typical phrases are 'bequemer Sessel' (comfortable armchair), 'gemütlicher Sessel' (cozy armchair), or 'Ledersessel' (leather armchair).
Für meine Leseecke wünsche ich mir einen gemütlichen Sessel mit hoher Lehne.
For my reading corner, I would like a cozy armchair with a high back.
Be careful: the meaning of 'Sessel' (armchair/chair) differs by region. In Austria and parts of southern Germany, the word often refers to a simple chair, for example at the dining table.
In Wien fragt man im Restaurant vielleicht: „Ist dieser Sessel hier noch frei?“
In Vienna, someone might ask in a restaurant: 'Is this chair here still free?'
chair / stool
'der Stuhl' and 'der Sessel' are both pieces of seating furniture for one person. In standard German, a 'Stuhl' is usually simpler: it has a seat, legs, and often a backrest, but normally no soft upholstery. A 'Sessel' is more comfortable, upholstered, and often has armrests. The words can be confused because you sit on both of them. In addition, in Austria and parts of southern Germany, 'Sessel' often simply means 'Stuhl' (chair). In normal standard German, 'Stuhl' is more likely to be used at a table, while 'Sessel' is more likely to belong in the living room for relaxing.

Nach der Arbeit setzt sich Maria in den weichen Sessel und liest ein Buch.
After work, Maria sits down in the soft armchair and reads a book.
Für die Feier morgen stellt mein Freund noch zehn Stühle in den Garten.
For the party tomorrow, my friend is putting ten more chairs in the garden.