The easiest way is to set ShowGridLines
property of your Grid to True
ShowGridLines="True"
There are other options, depending on how you want the outlines be displayed exactly. Some of those options are explained in this SO post.
UPDATE :
Since you develop on Win RT platform which doesn't have ShowGridLines
property, you should make outlines on your own. The minimum effort I can think is to add Rectangle for each Row spanning through all columns, and add Rectangle for each column spanning through all rows. That way will need less Rectangles than if you create it for each cell as shown in above link. Check this other SO post for example. And example for your case :
<Grid.ColumnDefinitions>
<ColumnDefinition Width="2*"/>
<ColumnDefinition Width="7*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="1*"/>
<RowDefinition Height="1*"/>
<RowDefinition Height="1*"/>
<RowDefinition Height="1*"/>
<RowDefinition Height="1*"/>
</Grid.RowDefinitions>
<TextBlock x:Name="lText1" Grid.Column="1" Grid.Row="0" Text="Definition 1" FontSize="33"/>
<TextBlock x:Name="lText2" Grid.Column="1" Grid.Row="1" Text="Definition 2" FontSize="33"/>
<TextBlock x:Name="lText3" Grid.Column="1" Grid.Row="2" Text="Definition 3" FontSize="33"/>
<TextBlock x:Name="lText4" Grid.Column="1" Grid.Row="3" Text="Definition 4" FontSize="33"/>
<TextBlock x:Name="lText5" Grid.Column="1" Grid.Row="4" Text="Definition 5" FontSize="33"/>
<!-- Horizontal Lines -->
<Rectangle Grid.ColumnSpan="2" Height="1" VerticalAlignment="Bottom" Fill="Black"/>
<Rectangle Grid.Row="1" Grid.ColumnSpan="2" Height="1" VerticalAlignment="Bottom" Fill="Black"/>
<Rectangle Grid.Row="2" Grid.ColumnSpan="2" Height="1" VerticalAlignment="Bottom" Fill="Black"/>
<Rectangle Grid.Row="3" Grid.ColumnSpan="2" Height="1" VerticalAlignment="Bottom" Fill="Black"/>
<Rectangle Grid.Row="4" Grid.ColumnSpan="2" Height="1" VerticalAlignment="Bottom" Fill="Black"/>
<!-- Vertical Lines -->
<Rectangle Grid.RowSpan="5" Width="1" HorizontalAlignment="Right" Fill="Black"/>
<Rectangle Grid.Column="1" Grid.RowSpan="5" Width="1" HorizontalAlignment="Right" Fill="Black"/>