First, here's how not to do it.
"[Name of the thing] is defined by Smith (2001) as ..."
I know this is how many people define terms, but it's not always good to put the name of the thing you are defining at the start of the paragraph. If this is the structure you use every time, it quickly becomes repetitive and dull.
Instead, set up a context or situation that gets the reader interested.
The exact definition of ... is the cause of some disagreement in the field. Perhaps the most influential definition is that of Smith (2001), who described it as ..."
This places Smith's definition within a context. The reader knows that it's influential but that there's some disagreement, and it's perfectly set up to then discuss other definitions.
One of the key criticisms of Smith's definition is that it does not take into account... To address this, Jones (2010) proposed...
Then you can say which definition you are using for your work.
If there is no disagreement and you're describing an accepted term, you can approach it in a slightly different way.
If it's a technique, state what problem it solves;
Until the late 1980's, there were no practical methods for determining ... This was until the development of ..., which uses...
Again, this puts the term you're defining into a context. This is one of the easiest ways to bring your writing to life.