"define the word" is correct and usable in written English.
You can use this phrase when you are asking someone to describe the meaning of a certain word. For example: "Can you define the word 'askew'?".
"Can you define the word 'push'? . . .
Can you please define the word "slag"?
One night, he asked me to define the word "adhere".
"Lisbeth, can you define the word friendship for me?" he asks.
But its seriousness, he said, depends on how you define the word "building".
I tried to define the word "roller" without sneaking a look in the dictionary.
Journals, textbooks and reference works use pandemic in discussing certain diseases, but rarely define the word.
"The IAU unfortunately should never have tried to define the word planet".
Three-quarters answered "I don't know" when asked to define the word "Aryanisation".
Kafka's extrapolation from his experience of paternal authority and naysaying in works such as "The Trial" define the word "Kafkaesque".
Whether everything in this six-artist show is really subtle may depend on how you define the word.
