<div>
Assuming that the acronym's definition is a part of the object that is bound the the list box row, perhaps something like: 
 
Form1.vb -
<pre><code class="code-10 nolinks" id="code-20-42073974-1">Public Class Form1
 Private lastHoveredIndex As Integer = -1

 Private Sub OnLoad(sender As Object, e As EventArgs) Handles MyBase.Load
 ListBox1.DataSource = (From i In Enumerable.Range(0, 20) Select New Acronym() With {.ID = i, .Acronym = String.Format(&quot;ACR{0}&quot;, i), .Definition = String.Format(&quot;Definition for ACR{0}&quot;, i)}).ToList()
 ListBox1.DisplayMember = &quot;Acronym&quot;
 End Sub

 Private Sub OnMouseMove(sender As Object, e As MouseEventArgs) Handles ListBox1.MouseMove
 Dim newHoveredIndex = ListBox1.IndexFromPoint(e.Location)
 If (lastHoveredIndex &lt;&gt; newHoveredIndex) Then
 lastHoveredIndex = newHoveredIndex
 If (lastHoveredIndex &gt; -1) Then
 ToolTip1.Active = False
 ToolTip1.SetToolTip(ListBox1, CType(ListBox1.Items(lastHoveredIndex), Acronym).Definition)
 ToolTip1.Active = True
 End If
 End If
 End Sub
End Class

Class Acronym
 Public Property ID() As Integer
 Public Property Acronym() As String
 Public Property Definition() As String
End Class
</code></pre>
Form1.Designer.vb -
<pre><code class="code-10 nolinks" id="code-20-42073974-2">&lt;Global.Microsoft.VisualBasic.CompilerServices.DesignerGenerated()&gt; _
Partial Class Form1
 Inherits System.Windows.Forms.Form

 'Form overrides dispose to clean up the component list.
 &lt;System.Diagnostics.DebuggerNonUserCode()&gt; _
 Protected Overrides Sub Dispose(ByVal disposing As Boolean)
 Try
 If disposing AndAlso components IsNot Nothing Then
 components.Dispose()
 End If
 Finally
 MyBase.Dispose(disposing)
 End Try
 End Sub

 'Required by the Windows Form Designer
 Private components As System.ComponentModel.IContainer

 'NOTE: The following procedure is required by the Windows Form Designer
 'It can be modified using the Windows Form Designer. 
 'Do not modify it using the code editor.
 &lt;System.Diagnostics.DebuggerStepThrough()&gt; _
 Private Sub InitializeComponent()
 Me.components = New System.ComponentModel.Container()
 Me.ListBox1 = New System.Windows.Forms.ListBox()
 Me.ToolTip1 = New System.Windows.Forms.ToolTip(Me.components)
 Me.SuspendLayout()
 '
 'ListBox1
 '
 Me.ListBox1.FormattingEnabled = True
 Me.ListBox1.Location = New System.Drawing.Point(13, 13)
 Me.ListBox1.Name = &quot;ListBox1&quot;
 Me.ListBox1.Size = New System.Drawing.Size(259, 238)
 Me.ListBox1.TabIndex = 0
 '
 'Form1
 '
 Me.AutoScaleDimensions = New System.Drawing.SizeF(6.0!, 13.0!)
 Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
 Me.ClientSize = New System.Drawing.Size(284, 261)
 Me.Controls.Add(Me.ListBox1)
 Me.Name = &quot;Form1&quot;
 Me.Text = &quot;Form1&quot;
 Me.ResumeLayout(False)

 End Sub

 Friend WithEvents ListBox1 As ListBox
 Friend WithEvents ToolTip1 As ToolTip
End Class
</code></pre>
Which produces the following output -<a href="https://filedb.experts-exchange.com/incoming/2017/03_w13/1154118/Capture.PNG" target="_blank" title="Capture.PNG (25 KB)"><img alt="Capture.PNG" class="file-block lazyload" style="max-width: 585px;" data-src="https://filedb.experts-exchange.com/incoming/2017/03_w13/1154118/Capture.PNG" /></a> 
-saige-
</div>

Capture.PNG

<div>
Hello, 
 
My apology for the late reply. 
 
How do you create the acronym definition and bound it to the list box row? 
 
Victor
</div>

<div>
With this:
<pre><code class="code-1 nolinks" id="code-20-42111486-1"> Private Sub OnLoad(sender As Object, e As EventArgs) Handles MyBase.Load
 ListBox1.DataSource = (From i In Enumerable.Range(0, 20) Select New Acronym() With {.ID = i, .Acronym = String.Format(&quot;ACR{0}&quot;, i), .Definition = String.Format(&quot;Definition for ACR{0}&quot;, i)}).ToList()
 ListBox1.DisplayMember = &quot;Acronym&quot;
 End Sub
</code></pre>
Which uses this class definition for Acronym:
<pre><code class="code-1 nolinks" id="code-20-42111486-2">Class Acronym
 Public Property ID() As Integer
 Public Property Acronym() As String
 Public Property Definition() As String
End Class
</code></pre>
 
-saige-
</div>

<div>
Hi, 
How do I build the part to display the definition? 
 
For example if mouse is over SN row to display &nbsp;&quot;Serial Number&quot;? 
 
Thanks, 
 
Victor
</div>

<div>
<div class="content wysiwyg-content">
Hi, 
&nbsp;I have a listbox &nbsp;with multiple rows containing acronyms, when I place a my cursor on a row, how do I display the definition of the row my mouse is over using VB.NET?, for example when place mouse on row containing NSC would like to see National Short Code 
Thanks, 
 
Victor
</div>
</div>

Hi,
 I have a listbox  with multiple rows containing acronyms, when I place a my cursor on a row, how do I display the definition of the row my mouse is over using VB.NET?, for example when place mouse on row containing NSC would like to see National Short Code
Thanks,

Victor

Help with  viewing definition of acronym when placing mouse on a row of a listbox

The .NET Framework is not specific to any one programming language; rather, it includes a library of functions that allows developers to rapidly build applications. Several supported languages include C#, VB.NET, C++ or ASP.NET.

.NET Programming

Visual Basic .NET (VB.NET) is an object-oriented programming language implemented on the .NET framework, but also supported on other platforms such as Mono and Silverlight. Microsoft launched VB.NET as the successor to the Visual Basic language. Though it is similar in syntax to  Visual Basic pre-2002, it is not the same technology,