The word وسط means the middle or center of something. Many times you will see it spelled two different ways:
Although they are very similar in meaning there is a slight nuance. The first one (وَسَط) means exactly in the middle. The second one (وَسْط) means somewhere in the middle, almost like a range.
A way to remember the difference is with the phrase:
إذا سَكَنَ تَحَرَّكَ وإذا تَحَرَّكَ سَكَنَ
(If the word has a sukoon [#2] then it has a range, and if it has a vowel [#1], then it is precise)
We hope this helps!!